Chapter: Animals

Characteristics of Animals

Animals form a monophyletic group, meaning all animals stem from a singular ancestor. So what makes an animals an animal? Well, all animals are multicellular and heterotrophic. That means they can not make their own food (like plants). They have to exist by eating other things: be it plant, fungi, protists, or even prokaryotes. And all animals can move under their own power. That is…unless they had a wee too much fun on a Saturday night. And all sponges except sponges have muscle cells and nerve cells. Sponges are a fairly strange group for that reason. However, you don’t have to worry about cutting them in two, because they can’t feel any thing. 

Diversification of Tissues

In addition to multicellular and heterotrophic, all animals have tissues. Tissues are specific groupings of cells that are integrated both in their structure and what they do, their function. For example all animals, except sponges (sponges are just weird), have epithelial tissues. These are a grouping of cells that cover a surface of the body of an organism or the cover of an organ. You might remember epithelial tissues from your studies of cellular interactions. Epithelial cells are held together with proteins connecting cells very tightly. In fact, these structures are called tight junctions. Tight junctions serve to create a water proof barrier between tissues.

Early in development, animals are known as embryos. And animal embryos differ in two main ways, which determine the final outcome of the adult. The most primitive are known as diploblasts. "Diplo" is Greek for "two." "Blast" is Greek for "embryonic tissue layer." So there are two embryonic tissue layers. These are also known as germ layers. The interior layer is called the endoderm (literally “the inner skin”), while the outer later is the ectoderm (literally “the outer skin”). Triploblasts have….you guessed it…three layers of embryonic tissues. It also has an inner endoderm, and an outer ectoderm. But it also has a middle layer known as the mesoderm. "Meso" stands for "middle," so it is the middle skin.

Each of these germ layers develop into specific parts of the body. For example, the ectoderm eventually becomes the skin and fat tissues that encompass the body. Whereas the endoderm develops into the digestive tract of animals. And this is true for diploblasts and triploblasts. So diploblasts are relatively simply organisms that are basically skin with a digestive system. Most animals that you think about are triploblasts. The mesoderm gives rise to all the inner organs of animals (other than the digestive tract. So in humans, that includes organs like the heart lungs, and even brain.

Diplobasts include some of the most primitive animals: the cnidarians, which include the jellyfish, corals, and sea anemones. The also include the ctenophores (also known as the comb jellies). As you can see, the animals are really nothing more than stomachs with skin. All other animals that you can think of are triploblasts.

Nervous Systems

All animals have a nervous system except, sponges. Jelly fish and comb jellies have a nerve net. A nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows these organisms to respond to physical contact. They may then detect food and other chemicals in a rudimentary way. Although the nerve net allows the animal to respond to its environment, it has trouble alerting the animal from where the stimulus is coming. For this reason, simple animals with nerve nets will typically respond in the same way to contact with an object, regardless of where the contact occurs. All other animals have a centralized nervous system, known as cephalization. This system combines masses known as ganglia, and tracts known as cords. Humans have one main ganglium, the brain, and messages are sent through our bodies through nerve cords. Some animals have more than one ganglia.

Body Symmetry

Animals have three different basic body plans. They can either be asymmetrical, meaning they have no symmetry. Some sponges are like this. However, all other animals are either radially symmetrical or bilaterally symmetrical.  Organisms that are radially symmetrical have more one plane of symmetry. Organisms that are radially symmetrical are the cnidarians (otherwise known as jelly fish) and the ctenophores (the comb jellies). The advantage of being radially symmetrical (vs. asymmetrical) is that you are equally likely to encounter food in any direction.

Nearly all animals that we see on Animal Planet are bilaterally symmetrical. This means that these animals only have one line of symmetry. In other words, half of the body is a mirror image of the other (but only in one direction). And typically, bilateral animals tend to be long and slender. The biggest advantage of this is that bilateral organisms tend to interact with their environment at one end. This allowed a concentration of ganglia, which is known as cephalization. In this way, there was a concentration of sensing and feeding structures. We know this is the head.

Evolution of the Body Cavity

A primitive group of triploblastic animals are known commonly as flatworms. And they are acoelomates. This means that they don’t have a coelom. A coelom is an additional opening in the mesoderm of a triploblastic. It is a container filled with fluid and completely lined by the mesoderm.  They serve two main functions: either as a container for oxygen and nutrients and they are important in hydrostatic movement, in organisms such as worms.

Hydrostatic Movement

Worms move by hydrostatic motion. This is due to the design of their body cavity. This is a cross section of a worm. The outer portion of the worm is its body wall. In the middle is its gut. Muscles attach to the body wall. And between the muscle and the gut is the coelom, which fills with liquid and allows hydrostatic movement. Let’s look at how they move. This means that their bodies are filled with fluid and the muscles in the body contract and relax in a pattern that allows the fluid inside the coelom to move back and forth. In this way, worms can move. Worms are wriggly. And the reason is because of how they contract and relax their muscles. They typically only contract one side of their body at a time. When this happens the body bends, because the fluid-filled cavity changes shape. Worms coordinate their muscles, moving fluid from one side of their body to another, allowing them to move.

 Animal Phylogenetics

Compared to protist and fungi phylogenetics, animal phylogenetics is far more straightforward. Animals’ closest living relative are protists known as choanoflagellates, and they really look nothing like any animal. So far we have discussed the most primitive animals. Porifera are sponges that don’t have any body symmetry, and lack different tissue types. You have also become familiar with Cnidaria and Ctenophora. These are the only groups of animals that have radial symmetry throughout their lifetimes, and are sometimes lumped together in a grouping called Radiata.

Bilateria

All of the other organisms that we have on Earth are in another group known as Bilateria. All animals in this group are triploblastic with a coelom….and have bilateral symmetry….duh!!! This group is broken into to other group: the protostomes and the deuterostomes. And these two groups differ in two ways. In protostomes, the mouth develops before the anus; whereas in deuterostomes, the anus develops before the mouth.

The second difference involves how the coelom forms. In protostomes, the mesoderm hollows from the inside to form the coelom; whereas in deuterostomes, the mesoderm pinches off to form the coelom. Animals in the Bilateria have what has been called a tube within a tube design. And a worm is a great way to think about this. When we see a worm, we see that it is tube-shaped. The outer part of the body wall looks kind of like a slimy straw. And within that body cavity is yet another tube known as the gut. And in essence, humans have this same body plan…with some minor alterations. We have our outer body (albeit with arms and legs), and with out body we have a sophisticated gut, which we know as the digestive system.

Choanoflagellates vs. Spronges

Choanoflagellates are protists and are considered to be the closest living relatives to animals. Choanoflagellates are unicellular; whereas all animals are multicellular. There closest animal cousin is the sponges. Both of these organisms are sessile, meaning that they are physically attached to a substrate, like a rock (or coral). Choanoflagellates feed by attaching themselves to a substrate and catching food particles moving through the water. Sponges are multicellular. And they feed with cells that structurally look a lot like choanoflagellates. In fact, they are called choanocytes. Where a choanoflagellate catches food particles in the open water, a sponge is shaped like a vessel,  in which water comes into it and the choanocytes capture food particles on the inside of the vessel. This allows for a more efficient capture of food particles. It is the first tube-within-a-tube design. 

Aceolomates

Of all the bilaterally symmetrical animals, the next grouping has to do with the coelom: whether it has it or not. Most animals have a coelom, including you. However, a more primitive design lacks a coelom, and the group is known as acoelomorpha, which literally translates to a morphology without a coelom. These organisms biggest advantage over radially symmetrical organisms is that they began to develop a head on one end of their body. This process is known as cephalization.  All other animals on earth are known as coelomates. And they all have a coelom.

Coelomates

This group is broken up into two other groups: the protostomes and the deuterostomes. Remember the protostomes are the ones that the first pore to develop becomes the mouth while in deuterostomes the first pore becomes the anus. Protostomes have another major split within them. One group, the lophotrochozoa grow by extending the size of their skeletons, like worms and mollusks. However, the coelom was somehow lost in flatworms. This is another example of an unparsimonious evolutionary adaptation. The other group, the Ecdysozoa, grow by shedding a tough exoskeleton in order that their soft bodies may expand. Insects are the best known of this group.

Sensory Organs

One of the biggest advantages of cephalization is the organization of sensory organs in one area. In the heads of these animals, many organs have developed to analyze and react to different senses: touch, smell, and taste. And one of the most important sensory organs for many animals is the eye. Here we can see living examples that demonstrate the evolution of the eye. Eyes begin as a group of light-sensitive cells connected to a neuron. These are known as eyespots, and seen in animals such as the limpet. These structures can detect light and dark, but lack an real sophistication. Nautilus have a fluid filled eye, but no lens. This allowed them to detect a direction of light and dark, but that was about it. A major development came when the lens came around. A lens is a dense glassy organic tissue that allows animals to focus light onto light sensitive cells. This adaptation was the first to allow animals to see shapes…however, the shapes would appear very blurry by our standards because the lens is fixed. It doesn’t have the ability to move.

The most sophisticated visual organ is the eye, and it has a lens that has the ability to change shape. This allows organisms such as the octopus (and humans) to be able to focus on an object at one distance and then look at an object at a different distance and be able to refocus it. This is like what a camera with an autofocus is doing. You’ll see it zoom in and out until it finds the right focal point. The lens is changing shape to do this; and that is exactly what happens in our eyes.

Suspension Feeders

In the next slides, we will discuss different feeding strategies. Many marine species are filter feeders. And in fact, this is thought to be the first feeding strategy of animals. Most suspensions feeders have gills that filter food particles from the water. Sponges, coral, and clams all feed in this manner. And you might be surprised to learn that the largest organisms on earth, the whales, eat some of the smallest (plankton). And they do so, by filter feeding through a specialized organ known as a baleen. It is like a really fine comb. 

Deposit Feeders

It is thought that the next feeding strategy to develop was deposit feeding. This is where an organism eats it way through a substrate (like soil) ingesting it and extracting food from within their gut. These are not terrestrial (not aquatic) and are common in the soil on land or in the sea floor. They typically have very simple mouth parts, basically just an opening. And all organisms with this feeding strategy are worm-like.

Fluid Feeders

Butterflies, hummingbirds, and flies are fluid feeders. They eat by sopping up liquids. Many of them have piercing mouth parts, so that they may puncture an outer layer so that they may get to the liquid innards of their food source. Flies do this. That is why a horsefly bite hurts. But the fly actually mops up the blood with a tongue.

Mass feeders

We are mass feeders. We take chunks of food into a mouth, and digest them inside of a digestive system. Many animals are mass feeders, and the structure of the mouth parts correlates with the type of food that they eat. Mammals that feed on grass all have front teeth that nip the grass and molars in order to chew it before it goes into the stomach. Snails have a different approach. They scrape parts of a leaf off with a radula. It is kind of like using a razor to remove hair.

 Reproduction

Reproduction in animals is highly variable. It can either be asexual, where a new organism simply buds off an existing organism. The polyp in the left picture is an example of this. However, most of animals reproduce by sex. However, the act of fertilization can happen internally, as in dragonflies (or humans) or externally, as in some fish or this coral.

 Development of Embryos

And of the animal that fertilize internally, there are 3 different ways in which the embryos develop. Humans are viviparous. This means that females retain eggs within their body, and give birth to live young. However, humans (and mammals, in general) are an oddity. Most animals lay eggs outside of their bodies to develop. Birds do this; but so to reptile, amphibians, and fish. This is known as oviparous.

Some organisms develop embryos in a hybrid fashion. The retain their eggs after fertilization for a short time.  They give birth to live, but highly undeveloped, young.  And later in their development they are fed yolk from outside the egg. Ovoviviparous animals are similar to viviparous species in that there is internal fertilization and the young are born live, but differ in that there is no placental connection and the unborn young are nourished by egg yolk; the mother's body does provide gas exchange, but that is largely necessary for oviparous animals as well.

Life Forms

Many insects have three different life forms. They start their lives as larvae, which look completely different than their adult form, and they typically eat completely different foods. Juveniles look and act like adults, but they differ in that they are not sexually mature. Adults are fully reproductive, meaning they have the ability to produce offspring.

Metamorphosis

The transition from one life form to another is known as its metamorphosis. And there are two different forms of metamorphosis. In hemimetabolous metamorphosis, the juvenile (known as a nymph) looks like a small version of the adult, as is the case in aphids. In holometabolous metamorphosis, the juvenile (known as a larva) looks different than an adult. And the transition from lava to adult occurs when the larva encases itself inside of a pupa, which it then develops. Mosquitos (and butterflies) go through holometabolous metamorphosis.

Porifera

The remainder of the slides are a survey of the most primitive animal groups. The Porifera are also known as sponges, and are the most primitive animals group. They are suspension feeders, and reproduce almost completely asexually. Larvae are able to move around to find open substrates to grow. But much like a plant, once they begin to grow, they are committed to growing in a single location.

Cnidaria

Cnidaria are radially symmetrical diploblasts, and they have three distinct body types. They all start out as larvae, and are able to move via jet propulsion. Some maintain the ability to move via jet propulsion, such as the jellyfish. Others such as coral are sessile, meaning they are attached to a substrate. Once attached they can reproduce asexually via polyps. Polyps are basically buds of animals that can grow into other adults. The jellyfish as an adult is a free-floating, sexual organism (and is known as a medusa).

Ctenophora

The ctenophores are a lesser known group of organism, which are also radially symmetrical diplobasts. They are transparent and gelatinous like jellyfish, but are not jellyfish. They eat plankton and are able to move by cilia, which is a comblike row of hairs along the outer edge of its body.

Acoelomorpha 

Acoelomorpha may be the strangest group of animals. They are the most primitive bilateral triplobastic animal, but they lack a coelom. So they represent the most primitive organisms that live today that have two important adaptions present in all the other animals. They are bilateral, and they are triploblastic. Most of them live in mud and sand in marine environments and also move by beating cilia.

Protostomes

Protostomes are the most abundant animals in the world, in terms of species diversity. Many of the groups of protostomes are worm-like and live in marine or freshwater environments. However, the most abundant number of species are not worm-like. These are the arthropods, which include the insects and crustaceans (like lobsters). And these species are very distinct and complex and can live in a large range of environments.

 Protostome Phylogenetics

The protostomes are split into two main groups: the Lophotrochozoa and the Ecdysozoa. And members within these groups can look really different. The one difference that they have between them is the way they grow. The Lophotrochozoa all grow by extending their skeleton. So worms stretch in order to grow. And clams just add layers to their shell and stretch their bodies to grow. In this way, these organisms grow incrementally, not in stages. The Ecdysozoa all shed their exoskeleton in order to expand their bodies. This process is known as molting. The best known example of this are the insects. Insects are trapped inside a fixed body due to their exoskeleton. In order to grow in size, insects have to leave their exoskeleton behind. They then expand in size and then form another exoskeleton.

Lophotrochozoa 

The Lophotrochozoa also have a specialized structure that rings their mouth that allows filter feeding. It is known as the lophophore. It is a ringed structure that pulls water and food particles into the mouth and the gut. This is how these organisms feed, via suspension feeding.

Protostome Body Plans

Protostomes are all triploblastic bilaterally symmetrically animals. Some are wormlike. Others aren’t. Those that are wormlike have a well developed coelom. This provides a space for fluids to circulate in order to provide a hydrostatic skeleton for worm movement.

Wormlike body plans

The wormlike body plans can be differentianted into groups by how they eat. The Spoon worms, officially known as the Echiurans, are segmented worms. And the Echiurans feed by using a specialized mouth part that extends from the mouth, in order to capture food particles. This is known as a proboscis. Food particles are trapped by hairs and mucus in the proboscis and the cilia move the food particle closer to the mouth to be digested.  It is a lot like the Sarlacc from Star Wars. You know the one, where Jabba the Hut captures Luke Skywalker and the gang and makes them walk the plank into the big toothy thing.  The pripulids are also known as penis worms. I DIDN’T MAKE THAT UP. THAT IS WHAT THEY ARE CALLED. Anyway, these worms eat with a toothed throat that can turn inside out in order to capture food and the retracts. Here is another movie reference. The ribbon worm’s mouth part is a lot like the little mouth of the monster from the movie Alien. The nemerteans are known as ribbon worms and their mouths have a barbed-tipped proboscis that extends to ensnare they prey and then retracts into its mouth.

Arthropod Body Plans

Arthropods include insects and are protostomes too, but with a completely different body plan. The biggest difference is that they have three specific and differently structured body parts: the head, the thorax and the abdomen. They also have jointed limbs (worms don’t). And the have a chitinous exoskeleton. And instead of moving hydrostatically, they move with muscles directly.

Mollusc Body Plans

Molluscs are also protostomes; and they have a totally different body plan than worms or arthropods. These include snails, and slugs. Molluscs moves with a large muscle at the base of their bodies. On top of that foot is its internal organ in a structure known as its visceral mass. And the part that covers the visceral mass is known as the mantle. And in some taxa, the mantle secretes calcium carbonate to produce a shell. You know them as snails. Slugs however, don’t make shells.

 Adaptations for Feeding

Protostomes have evolved every kind of feeding mechanism known in animals. They are filter feeding that feed via suspension feeders. They are are also deposit, liquid and mass feeders. And they have a very diverse way of getting their food. They can pierce, suck, grind, bite, chew and cut.

 Adaptations for Moving

In the Arthropods, jointed limbs were adapted. This allows for those organisms to be able to move very efficiently on land. Wings however, have been the most important adaptation for all protostomes. And the evidence that supports this is that 2/3 of all multicellular species are winged insects. And the mollusk can crawl with its pseudopod, which literally translates to its fake foot. Some aquatic organisms have developed jet propulsion. That is when an organism like a squid fills a sac-like structure called a mantle and forces it out of a siphon causing the animals to move in one direction.

 Adaptations is Reproduction

Animals have developed several different ways of reproducing and protostomes do all of them. Some organisms, like the Planaria in the photo on the left, just split into two. Others, like a tapeworm, can grow from segments that split from original organism. And in some insects, eggs that haven’t been unfertilized will continue to develop and grow into adults.

Sexual reproduction is the primary form of reproduction in all animals, including the protostomes. Sessile organism, that is ones that are always attached to a substrate, fertilize by releasing gametes and allowing them to mix randomly. This is common in coral reef communities. Organisms that move around almost exclusively reproduce sexually with internal fertilization.

Survey of Protostomes

Buckle your seatbelts. We are going to go through a survey of the major protostome groups. From the rotifers to the insects.

 Rotifers

Rotifers are important components of plankton in fresh and brackish water. They have a cluster of cilia at one end known as the corona. In many species, cilia in the corona beat making suspension feeding possible, forcing food particles into the gut.

Flatworms

The phylum Platyhelminthes are very diverse and includes all the flatworms.  In general, they have a very broad and flattened body shape with only one opening for ingesting food and elimation of wastes.

Turbellarians

Of the flatworms there are three main types. Turbellarians are free living flatworms that commonly inhabit coral reef ecosystems. They typically hunt protists and other small organisms. They can also serve as scavengers.

Cestodes

Cestodes are parasites that we know as tapeworms. They don’t have a mouth nor a digestive system, but they simply absorb nutrients into their body by diffusion straight from their host. These are nature’s ultimate moochers.

Trematodes

The last group of flatworms are the trematodes. They are also parasites, that bite off and ingest their hosts’ tissues. So they have their own digestive tract, whereas the tapeworms don’t.

Annelida

The next group we will talk about are Annelida. These are also known as the segmented worms.

Polychaeta

It is thought the oldest group of annelids are the polychaeta. These are segmented worms that have parapodia. Parapodia are foot-like appendages with chaetae. Chaetae are very unique bristlelike appendages that come from out of these organisms. Members are of the polychaeta are not typically very well known by the public. So literally the polychaeta are organisms with many bristle-like feet.

Oligochaeta

The oligochaeta literally translates to a few bristle like feet. And this group includes the earthworms. All of these organism don’t have the parapodia (the extension from the body), but they do have very reduced chaetae.

Hirundinea 

The Hirundinea include the leaches and members of this group have completely lost their parapodia and their chaetae. Maybe they should be renamed Achaetae.

Mollusca

Mollusc are the next group I will tell you about. And they have four different groups within the overall group.

Bivalves

Bivalves all have two shells attached by a hinge. And they are all suspension feeders. On the left is a picture of a clam. Clams bury themselves in to the sand and filter feed with just their mouthparts exposed to the water. Oysters and mussels attach themselves to a substrate and filter feed that way. And scallops are able to move around.

Gastropods 

Gastropods are marine snails and slugs. And you can always tell them because they have a large, muscular foot that allows them to move. They also feed by radula, which is a mouth on the head part of the mouth. Can you imagine if you foot had a mouth? Weird…huh?

Chitons 

Chitons are a weird group. They are a lot like the gastropods in that they have a muscular foot and feed by a radula. However, they all have 8 unique plates that they use for protection; whereas a snail just has a single shell.

Cephalapods

Cephalapods include the nautilus (in the top picture), cuttlefish, squid and octopus. They all have a very well-developed head (including a beak). In fact, in octopi the beak is the only hard substance in the whole organism. Cephalod feet have been modified to form tentacle that they can use for grasping. And these are the most intelligent of all the protostomes. In fact, I consider this group the great apes of the protostomes. I’ll bet you didn’t think that an octopus was so closely related to a clam…did you?

Ecdysozoa

So now we will leave the lophotrochozoa and look at the other major group of the protostomes, known as the Ecdysozoa.

Nematoda

Nematodes have successfully adapted to nearly every ecosystem from marine to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts, oceanic trenches, and within the earth's lithosphere. They represent, for example, 90% of all life forms on the ocean floor. Their numerical dominance, often exceeding more than 1 million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity in lifestyles and their presence at various trophic levels point at an important role in many ecosystems. Their many parasitic forms include pathogens in most plants and animals (including humans).  

Tardigrada 

The tardigrades are commonly known as the water bears. And they are a very small, water-dwelling animals. They have a segmented body with eight legs. Their name means slow walker and the reason they are called water bears is that they way they walk are reminiscent of a bear’s gait. Tardigrades are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures of close to absolute zero (−273 °C, temperatures as high as 151 °C (304 °F), 1,000 times more radiation than other animals, and almost a decade without water. Since 2007, tardigrades have also returned alive from studies in which they have been exposed to the vacuum of outer space for a few days in low earth orbit.

 Onychophora 

The velvet worms are not group of organisms that look a lot like catepillars. Onychophora — literally translates to "claw bearers”. These obscurely segmented organisms have tiny eyes, antennae, multiple pairs of legs and slime glands. They have variously been compared to worms with legs, caterpillars and slugs. Most common in tropical regions of the Southern Hemisphere, they prey on smaller animals such as insects, which they catch by squirting an adhesive slime. In modern zoology, they are particularly renowned for their curious mating behavior and for bearing live young. They are becoming increasingly popular as pets due to their bizarre appearance and eating habits. The females of many species are fertilized only once during their lives, which leads to copulation sometimes taking place before the reproductive organ of the females are fully developed. The transferred sperm cells are kept in a special reservoir, where they can remain viable for longer periods. 

Arthropoda 

An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropoda translates from Greek meaning  "jointed leg"), and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species, and are one of only two animal groups that are very successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long.

Myriopoda

The myriopoda are a group that includes the millipedes and centipedes. Millipedes are detritovores, meaning that eat decaying vegetation or other organic matter. And millipedes have two leg pairs per segment. Centipedes are very effective predators and have only a single pair of legs per segment.

Insecta

Insects  are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all known living organisms. The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the different life forms on Earth. Insects may be found in nearly all environments, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.

Chelicerata

The chelicerate body plan consists of two tagmata, the cephalothorax and the abdomen, except that mites have lost a visible division between these sections. The chelicerae, which give the group its name, are the only appendages that appear before the mouth. In most sub-groups they are modest pincers used in feeding. However, spiders' chelicerae form fangs which in most species are used to inject venom into their prey. Marine chelicerates have gills, while the air-breathing forms generally have both book lungs and tracheae. In general the ganglia of living chelicerates' central nervous systems fuse into large masses in the cephalothorax. Most chelicerates rely on modified bristles for touch and for information about vibrations, air currents, and chemical changes in their environment. The most active hunting spiders also have very acute eyesight.

Crustacea 

Crustacea are predominately a marine group of arthropods, including lobsters, shrimp and crab. Like the chelicerata, they have two tagamata. However they do have antennae. They also have a carapace, which is a platelike exoskeleton that protects the cephalothorax.

Deuterostomes

The deuterostomes are some of the most morphologically complex of all the animals. They are a monophyletic group and are all are very similar in how their embryos develop. And just like the protostomes they are a very variable group. They include four phyla. Star fish and sea urchins make up the Echinodermata. And all the vertebrates that you know and love make up the chordate. Two other phyla are less known. They are both wormlike. One group is known as the acorn worms, and form the phylum hemichordata. The xenoturbellida includes  wormlike organisms that have only been discovered recently. 

Echinodermata

Echinodermata Body Plan

So you might be thinking….wait!!! Starfish are radially symmetrical. Should they be more closely related to the other radially symmetrical groups, the Cnidarians and Ctenophorans. … I knew you were wondering that. Well, they are not. They are actually in the bilateral group of organisms. But you really only see that when they are larvae. Their larvae are bilateral. But as they develop they become radially symmetrical. They are truly strange. The starfish and echinoderms both have an endoskeleton This is a hard structure, just under the skin of the animal. It is made from calcium carbonate, the same stuff that is in shells, and it is a structure  that provides protection and support.

Echinoderms also have another very unique morphological feature known as the water vascular system. This is a series of branching, fluid filled tubes. These tubes fill and form a hydrostatic skeleton similar to worms. The water vascular system is also involved in movement. In starfish, the vascular system runs alone the arms of the starfish. Biologists call the arms tube feet. And the vascular system attaches to specialized structures that end outside of the tube feet. These are called podia, and literally translates to feet. As the podia extend and contract in a coordinated way, they alternatively grab and release a substrate, which allows them to move in a specific direction.

Podia are not only essential for moving they are also how members of the Echinodermata capture their food. Some members of the Echinodermata are very efficient predators in tidal zones of oceans and brackish waters. They can use their podia to pry open bivalves, such as clams. They then extrude their stomach through an opening in middle of their body and digest the clam in its shell. Starfish aren’t so cute know…are they? Other members of the Echinodermata use their podia to capture food particles in the current, and little hairs called cilia sweep the food into the mouth of the organism. This is known as suspension feeders and feather are a well known example of suspension feeding echinoderms.

Echinodermata lineages

Echinodermata have five main lineages. Crinoidea includes the feather stars and sea lilies. These are sessile suspension feeders….and they are really pretty. Brittle stars and basket stars make up the lineage Ophiuroidea. And they have 5 or more arms that radiate out from a central disc. But they use these arms to suspension feed. They capture food particles and move them toward their mouth via cilia. Sea stars (I always call them star fish) make up the lineage Asteroidea and they have bodies with 5 or more arms (some species have up to 40). Unlike the brittle stars, the sea star’s arms are not set off by clear, joint-like articulations. Sea stars are predatory. In fact, they are like the lions of the tide pools. They move by organizing the movement of their tube feet and podia. And they have sex with their arms. Strange….very strange indeed. Sea urchins and sand dollars make up the lineage Echinoidea. Compared to the rest of the group, the Echinoids are the only ones that have spherical bodies. Sea urchins are spiny echinoids that crawl using their spines. They constantly graze on algae and kelp. So if sea star are the lions. The echinoids are the gazelles of the tide pool. Whereas sand dollars have disk-shaped bodies and are suspension feeders. Maybe the strangest group of Echinodermata is the sea cucumbers. They make up the group Holothuroidea. They are sausage-shaped animal that suspension or deposit feed with aid of modified tube feet arranged in a whorl around their mouth.

Chordata

Vertebrate animals are chordates; but not all chordates are vertebrates. All members of the chordate, have a hollow nerve cord that runs the length of their body. This is like a nerve superhighway that connects the different part of the body. They also all have a notochord, which is a supportive yet flexible rod. In vertebrates, this notochord has developed to become the vertebral column, also known as the back bone. And as some point during their development all members of the chordata have openings in their throats called pharyngeal gill slits. These allow the organism to breathe in an aquatic environment. Members also all have a tail, at least at some point in their lives. You might be thinking…wait…humans are chordates and we don’t have gill slits or tails. And you are right. However, in the early development of the fetus humans do in fact have both of these structures.

Cephalochordata

Cephalochordata are better known as lancets. They resemble fish (but aren’t) and are small suspension feeders that can move around. And there are an important link between invertebrate and vertebrate organisms. Their notochord is a flexible, yet supportive rod and is the first known evolutionary endoskeleton. 

Urochordata 

The Urochordata are the sea squirts. This is a marine group of organisms that all have a U-shaped gut with two siphons that they use in the process of suspension feeding. One opening brings water and food particle in and the other expels water and waste.  They are also unique among the chordates because the have an exoskeleton coat, called a tunic. This group of organisms do not have a vertebral column, so they are not considered vertebrates. 

Chordata: Vertebrata

What makes a vertebrate a vertebrate is that their nerve cord develops into spinal cord. Another thing that they all have in common is that all vertebrates that gill slits in their early development. You can see these slits in organisms as different as fish, reptiles, birds, and even humans. In aquatic species, these pharyngeal gill slits become actual gills. While all vertebrates have a spinal cord and pharyngeal gill slits, there have been several innovations that have occurred in the evolution of vertebrates. One of the first was the development of a bony exoskeleton. This was the first appearance of scale-like patterns in aquatic animals, and was the predecessor of fish and reptile scales we see today. But some of the first ones were really thick and made out of bone.

Jaws also developed in the vertebrates. This represented a new method of feeding, in which a mouth is connected to a hinge that allows it to open and close very precisely. This proved so effective that nearly all vertebrates to this day have a jaw.

Contrary to what you might think, not all vertebrates have a bony endoskeleton. This innovation came in during the Silurian and gave fish with the bony endoskeleton a major advantage in the water due their ability to swim much more quickly and precisely. It’s kind of like comparing the speed and handling of a 1950s station wagon and a modern Formula 1 car.

And perhaps one of the craziest evolutionary adaptations that ever occurred was when fish fins evolutionarily developed into limbs that could be used for crawling. The first known animals that crawled on land are known as tetrapods (literally translating to four legs).

Lungfish

There is still an animal on Earth today that resembles what the earliest fish to come out of water. It is called a lung fish, and it has really strong fins and lungs. So it has the ability to live in pools that dry up. The advantage of this is that the lung fish can move over land from pool to pool without suffocating.

Amniotic Egg

Once on land, another adaptation evolved that greatly improved the chance of an organism’s offspring of surviving. And they are the amniotic eggs. These are eggs that are membranes that have yolk inside of them in order to nourish and protect the embryo. In reptiles and birds, they lay them and allow them to develop. 

Placenta

Mammals retain their amniotic eggs within their bodies in a specialized organ that contains a placenta which gives oxygen and nutrients directly from the mother to the embryo. And mammals give birth to live young.

Hagfish and Lampreys

The most primitive vertebrates are the hagfish and the lampreys. Both of these organisms lack jaws, but the do have a nerve cord that is not hollow. Hagfish lack a vertebral column, where a lamprey has pieces of cartilage along the nerve cord. And they also differ by how they eat. Hag fish are scavengers and predators of small prey. Lampreys attach themselves to larger fish (like whales and sharks) and feed off them as they move.

Sharks, rays and skates

All of these organisms have skeleton made of cartilage, not bone. And if you compare them with the hagfish and lampreys, the cartilaginous fish also have jaws and paired fins for a more stream-lined swimming.

Ray-finned fish

The bony fish are also known as the ray-finned fish. Compared with the cartilaginous fish (like the shark) they all have fins supported by long bony rods, a bony skeleton. They also have an organ that they can fill with air in order to regulate their depth with minimal energy. This is known as the swim bladder. This group is arguably the most successful group of vertebrates, if you consider the number of species that they make up.

Lobe finned fish

Lobe finned fish differ from ray finned fish in that their bottom fins are not pointed (or ray shaped). They are lobe shaped…or you can think of them as not pointed.

Evolutionarily, these are really important organisms, because they are thought to be the link between the fish and the first animals to walk on land, the tetrapods. There are two groups of lobe-finned fish on Earth today. The lung fish are present in shallow ponds. And the coelacanth lives in the ocean. Coelacanths were thought to have gone extinct in the Late Cretaceous, but were rediscovered in 1938 off the coast of South Africa.  The coelacanth has been nicknamed a “living fossil”, because it originally was known only through fossils, long before the first discovery of a live specimen.

All of the remaining groups of animals we will discuss are four-legged animals, known as tetrapods. And we will start with the amphibians.

Amphibians

Amphibians include frogs, toads and salamanders. And these groups have several things in common. Even though they have mouths they breathe through their skin. They are all carnivore, mostly eating small animals, especially insects. They all lay eggs in water and have four limbs. The one weird exception are the caeclians. These look like worms, but are actually amphibians that have evolutionarily lost their eyes and their limbs. Really weird. All the other vertebrates give birth in an amniotic egg. And we begin with mammals.

Mammals

All mammals have three things in common, which differentiates them from other vertebrates. They all have hair. They feed their young by mammary glands. And they are endothermic, meaning they produce their own body heat. Reptiles are exothermic. They rely on the environment for heat. There are three main groups of Mammals, the monotremes, the marsupials, and the eutherians.

Monotremes

Monotremes are a very rare group of mammals. In fact, only three species exist on Earth. The platypus is one of them. In fact, the platypus is so strange, that the first explorers to write about them were discounted because no one believed that such a strange animal exists. All monotremes are mammals that lay eggs and have leathery bills.

Marsupials

Marsupials are a group of mammals that basically give birth to a fetus, which then attaches to its mother’s nipple for most of its development. Marsupials are very common in Australia. However, there is one species native to the Americas: the opossum.

Eutherians

Eutherians include everything from whales to rabbits to humans. And these animals are differentiated from marsupials an monotremes, in that they have a highly developed placenta and give birth to highly developed young. Many animals in this group also have a long brooding period with their young. The combination of these factors have produced some of the most intelligent creatures on this planet.

Reptilia

Lizards, Turtles, crocodiles and birds all form the group Reptilia. And they all have scales with hard keratin. They also all have lungs, give birth in amniotic eggs and are ectothermic (meaning they are dependent on their environment for heat). Birds however, are endothermic.

Snakes & Lizards

The oldest group of extant reptiles are the lizards and snakes. Lizards and snakes make up a group known as the lepidiosauria. They all have elongated bodies and scaly skin. Lizards have jointed legs; whereas snakes are limbless. Some species of snakes actually have vestigial limbs that they use to hold onto their mate during intercourse. The picture shows one of these called a spur.

Turtles

Turtles and tortoises are a group of reptiles that have a shell of bony plates for protection. They all lack teeth and have a bony beak. Turtle are predominately aquatic; whereas tortoises are predominantly terrestrial.

Crocodiles

Crocodiles can be differentiated from the other groups of reptiles by having eyes and nostrils on the top part of their heads. This is an important adaptation for ambushing prey on the shoreline.

Birds

Birds are reptiles that have feathers, lightweight bones, and are endothermic.