Animals are classified by taxonomic considerations within the Kingdom Animalia according to qualitative traits, DNA sequences, and their combination: clades. Phylogenetic taxonomy is the most commonly used method today. It enables taxonomists to show a lot of information in their deceptively simple trees. There are 21 different animal phyla, but I will only talk about the most important 11, and I will neglect the Latin names because they are uninformative:
1) Sponges. These invertebrate animals lack nervous, digestive, and circulatory systems. They rely on water flowing through their bodies to absorb food and oxygen and to remove waste. They have the awesome ability to regenerate parts of their bodies that are broken off. In some bad cases the will produce survival pods called gemmules that remain dormant until things improve and then either form a new sponge or rejoin their parent.
2) Corals and Jellyfish. These invertebrate animals have only a layer or two of cells filled with a jelly-like substance called mesoglea. They also have venomous cells called cnidocytes that allow them to defend themselves and capture prey. They have decentralized nervous systems with collections of interneurons that connect to sensory and motor neurons. The central interneurons act as ganglia, or local coordination centers. Medusae swimming colonies have a sweet technique called the statocyst that allows them to orient themselves correctly and some species have rudimentary visual systems.
3) Flatworms. These invertebrates have bilateral symmetry and no body cavities so they have developed body types that allow them to get oxygen and nutrients through diffusion, which is why so many of them are microscopic. Some of them have a simple nervous system called a nerve net, which is a collection of non-encephalized interneurons that allow the worms to respond to physical contact and do some basic chemosensing.
4) Nematodes: Aka roundworms, these invertebrates live in either soil and water. According to rough numbers from Wikipedia, about 19% of them are parasitic. They have a complete digestive system and both a dorsal and ventral nerve cord, as well as chemosensory amphids and phasmids, which each contain sensory dendrites.
5) Moss animals. Aka sea mats, they build skeletons of calcium carbonate and are mainly located in warm, tropical water. They feed with a lophophore, which is basically just a bunch of ciliated tentacles around the mouth that reminds me of the worm in Star Wars: The Empire Strikes Back. They have a two lobe ganglion at the base of the lopophore that connects to the internal muscles and organs, as well as looping back to a nerve ring that innervates the tentacles of the lopophore.
6) Lamp shells. These are invertebrate marine animals with two valves that pull their valves apart via internal diductor muscles. They cannot move around, and are attached to the “substrate” via a pedicle. Their nervous system consists of a ring of nerves around the esophagus, from which nerves reach out to innervate the muscles, valves, and lopophore.
7) Mollusks. These are highly diverse invertebrates that have a single opening from their mantle used for both breathing and excretion. They have two main nerve chords with scattered ganglia acting as local control centers. These laterally paired ganglia often have commisures linking them. They generally have a pair of eyes, tentacles for both mechanical and chemical sensing, stotocysts in the feet as balance sensors, and a pair of osphradia, chemical sensors located in front of the lungs. Famous examples include snails, octupuses, and squids.
8) Segmented Worms. These invertebrate animals mostly live in wet environments and are bilaterally symmetric. They have a coelem, a closed circulatory system, and are obviously segmented. Their nervous system is pretty cool. There is a main nerve chord through the body that comes into contact with a lateral nerve from each segment. However, each segment is autonomous, so the local regions must coordinate to perform global actions such as locomotion. Examples of these are earthworms and leeches.
9) Arthropods. These invertebrates are characterized by jointed limbs and cuticles which is mainly made by a long polymer chain of N-acetylglucosamine: alpha-chitin. They are also segmented, and make up 80% of all living species, in large part because they are so succesful in dry environments. The stiff cuticles are penetrated by various touch sensors that feed to the nervous system, which often come in the form of setae. They also have chemical and pressure sensors, internal propioreceptors, and antennae to monitor temperature, moisture, and humidity. Ventral nerve chords run through each segment and form paired ganglion in each segment, and their brains are formed by the fusion of the ganglion from some of these segments and encircle the esophagus. Their nervous system has been described as “ladder like.” Examples include the arachnids, insects, and crustaceans.
10) Sea stars. These include the starfich, brittle stars, sea urchins, sand dollars, sea cucumbers, sea daisies, and the sea lilies. They have radial symmetry, and a mesodermal endoskeleton, meaning that their support system is within the tissue of the organism. Their nervous system is made up of interconnected neurons with assorted ganglia. There is a central ring of neurons around the mouth from which neurons radiate into each arm, but no actual brain. Some of them (the sea lillies) can’t move, but most of them can.
11) Chordates. These animals have a dorsal nerve chord, pharyngeal slits for feeding and/or breathing, an endostyle, a tail, and a notochord. This last one, used for axial support, is the most differentiating trait. If the notochord is present through the lifetime then the animal is an invertebrate, but if it is replaced in the adult stage by a vertebral column then the animal is a vertebrate. The invertebrates are sub-grouped into tunicates and lancelets, but the vertebrates are all in the same subphylum Vertebrata. The most important vertebrates are the jawless fishes, the cartilaginous fish, the bony fish, and then tetrapods, which are divided into the really famous classes: amphibians, aves, and mammals.
Inspired by Question #3 of CalTech’s 100 Questions for Cognitive Scientists: “Do you know how animals are classified? What are the major phyla? What different types of vertebrates exist?”
Ramel, G. “The Phylum Bracipodia.” Accessed January 2008. http://www.earthlife.net/inverts/brachiopoda.html.
Armstrong, WP. “Animal Phyla.” Accessed January 2008. http://waynesword.palomar.edu/trnov01.htm#porifera