Figure 1: Schematic plot of the most common Phyla with a fossil record and their relations, and the larger groups to which they belong.

Web information on the phylum.

"Sponges are the simplest multicellular animals. Because the cell is the elementary unit of life, the evolution of organisms larger than unicellular protozoa arose as an aggregate of such building units. [...] There are many advantages to multicellularity as opposed to simply increasing the mass of a single cell. Since it is at cell surfaces that exchange takes place, dividing a mass into smaller units greatly increases the surface area available for metabolic activities. […..] Thus multicellularity is a highly adaptive path toward increasing body size." (Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa)

Porifera are commonly referred to as sponges, and are thought to be the most 'primitive' multicellular animals, not far evolved from a colony of single cells. Fossil sponges are among the oldest known animal fossils (Late Precambrian), and present in Ediacaran fossil assemblages. Since then, sponges have been conspicuous in many fossil communities; there are more than 900 fossil genera, about 5000 living sponge species. Sponges' bodies are loose aggregations of cells, no endoderm or ectoderm cells (cells called mesenchym); no or radial symmetry;. There are no organs or true tissues; sponges have no mouth, but they have tiny pores in their outer walls through which water is drawn into canals and chambers. Cells in the sponge walls filter food particles from the water as the water is pumped through the body. Interior surfaces are lined with choanocytes, which are cells with a collar structure and a flagellum (hair); the moving 'hairs' create water currents. The flow of water through the sponge is thus driven by the beating of the flagella which line the surface of chambers and interconnecting canals. The animal has a gelatinous protein matrix called mesoglea, with skeletal elements. Skeletal elements are made of collagen (a protein), and calcareous or siliceous crystalline spicules (small spines), often combined with variously modified collagen (spongin) fibrils. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals. For instance, reactions to stimuli apparently local and independent; nervous system probably absent. All sponges are aquatic; most live in the sea, adults sessile and attached to substratum. Asexual reproduction by buds or gemmules (small clumps of cells), and sexual reproduction by eggs and sperm; free-swimming larvae.

Web information on the phylum.

"... many cnidarians are very effective predators that are able to kill and eat prey much more highly organized, swift, and intelligent. They manage these feats because they possess tentacles bristling with tiny, remarkably sophisticated weapons called nematocysts. A nematocyst is secreted within the cell that contains it. As it is secreted, it is endowed with the potential energy to power its discharge. […..] Achieving a velocity of 2 m/sec […..], it instantly penetrates its prey and injects a paralyzing toxin." Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Pages 440-441.

The Greek word "cnidos" (knidos) means "stinging nettle". Cnidarians are characterized by stinging cells (nematocysts or cnidocysts). These cells eject a barbed thread and often poison when the animal is disturbed. Cnidarians are bag-shaped, with a mouth but no anus, and usually tentacles around the mouth. In cross section, a cnidarian body consists of two cell layers: an ectoderm or outer layer, and an endoderm or inner layer. Between the two there is a layer of jellylike substance called mesogloea. Cnidarians may be either polyps, i.e., attached to the bottom like sea anemones and coral colonies or free-floating medusae (jellyfish). During the life of cnidarians, there may be alternation of free-swimming (medusae) and attached (polyp) stages, or only one stage may occur. Cnidaria have a radial symmetry around a longitudinal axis with oral (mouth) and aboral ends; no definite head. They may have a skeleton of chitinous, calcareous or proteinaceous compounds. There is a nerve net with symmetrical and asymmetrical synapses; some sensory organs; diffuse conduction, and a muscular system with an outer layer of longitudinal fibers and an inner one of circular fibers. No excretory or respiratory systems, no coelomic cavity. All cnidarians are aquatic; some live in fresh water but most are marine. Some forms (corals, jellyfish) live colonial. 'Colonial' animals reproduce (among other ways) by budding new parts asexually. These new additions contain functioning individuals, capable of feeding independently, yet they stay attached to the original individual; some individuals may specialize (e.g., in reproduction, defense), and then are fed by other individuals. In this case individuals in the colony thus function as organs for the colony, rather than as true individuals. The entire complex is called a 'colony'; the individual functional units can be called 'zooids', although 'polyp' is used for coral individuals. Colonial animals include corals, graptolites (see below; Phylum Hemichordata) and bryozoans (see below; Phylum Bryozoa). Additionally, there is debate about coloniality in sponges. Examples: Sea anemones, corals, jellyfish.

Information on Bryozoa; professional page for peope working with Bryozoa; a look at our local Connecticut Bryozoa.

Bryozoa are colonial animals, which are abundant in modern marine environments and also occur in fresh waters. A few to many millions of these individuals may form one colony. Colonies show a large variability in shape: encrusting, erect, arborescent, bilaminar sheets, narrow unilaminate branches, box to sac-shapes or short cylinders. Bryozoan colonies range from millimeters to meters in size, but the individuals that make up the colonies are rarely larger than a millimeter. They have left an abundant fossil record; in places, the skeletal remains are so abundant that the fossils become rock-forming. Vernacular names include 'sea mats', 'moss animals' or 'lace corals'. Bryozoan colonies have a superficial similarity with corals, but the anatomy of the bryozoan animal is much more complex. They have true organs, a mouth and anus (outside the lophophore), a body coelomic cavity, and a lophophore that they can withdraw inside the skeleton. Reproduction is by budding and sexual; many species have bisexual individuals. There are about 5000 living species, with several times that number of fossil species, but Bryozoa are not widely known or recognized; colonies may be mistaken for hydroids, corals, or even seaweeds. Some bryozoans encrust rocky surfaces, shells, or algae. Others form lacy or fan-like colonies that in some regions may make up the bulk mass of limestones. Living bryozoans are considered nuisances because more than 100 species grow on ships, causing drag; they may also foul pilings, piers, and docks. Fresh water species may form great jellylike colonies so huge they clog public or industrial water intakes. Bryozoans are presently studied intensively by biochemical companies, because they produce a large variety of chemical compounds, some of which have medicinal use; the compound bryostatin is tested as an anti-cancer drug.

Information on the Phylum ; brachiopodologists meeting page.

Brachiopods are marine animals that superficially look like clams, but that are quite different in their anatomy. They are not even closely related to the molluscs. Brachiopoda are (like Bryozoa, which belong together with the Brachiopoda in the Lophorate group of Phyla) not well known to the general public. They seem rare in today's seas close to the shore, but they are fairly common in cold water, either in polar regions or at great depths. There are about 300 living species of brachiopods. In the Paleozoic, however, they were extremely abundant, and diversified into a number of different morphologies and about 30,000 species, including reef builders. At the end of the Paleozoic, some 250 million years ago, they were decimated in the worst mass extinction of all time, the Permo-Triassic event.

Brachiopods have two shells, which in the group of the Inarticulata have no hinges, and consist of calcium phosphate, chitin and protein. Brachiopod shells are not left and right shells (as in molluscs), but front and back (ventral and dorsal); the plane of symmetry bisects each shell. The most famous Inarticulate brachiopood is the genus Lingula, which is a living fossils; it shells show no morphological differences from Devonian shells, and thus have not changed in morpohology in 450 million years. The larger group of Articulata has hinged valves of calcium carbonate. Brachiopods have no head, but do have a heart, complex digestive system with a mouth but no anus. The mouth is in the center of the lophophore. They have an intricate set of muscles for opening the valves, and many forms ate stalked (stalk is called peduncle). The coelom is complex in shape, and probably transports respiratory gases; they have no hemoglobin. The respiratory rate of brachiopods is one of the lowest in any known animal, and they can survive for very long periods without oxygen or food. There is a small nervous system, not well developed; sensory lobes can detect chemicals in sea water. All brachiopods are marine. Brachiopods are all solitary forms (no colonies), and reproduction is sexual.

Information on the Phylum (web site not yet finished). "Next to Arthropoda, the phylum Mollusca has the most named species in the animal kingdom: probably about 50,000 living species, not to mention some 35,000 fossil species discovered to date. The name Mollusca indicates one of their distinctive characteristics: a soft body." Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Page 499.

The molluscan body is bilaterally symmetrical, but asymmetric in some; it is unsegmented, usually with definite head. Molluscs have a muscular foot, variously modified but used for locomotion. The back of the body forms the mantle, which encloses the mantle cavity, is modified into gills or a lung, and secretes the shell (when present). The coelom is mainly present around the heart; the surface of the mantle usually is ciliated ('hairs'), and has mucous glands and nerve endings. Mollusca have a complex digestive system; a rasping organ (radula) is usually present; anus usually emptying into mantle cavity. There is an open circulatory system (with the exception of the class cephalopods), existing of a heart (usually three-chambered), blood vessels, and sinuses; there are respiratory pigments (based on copper, not iron as in vertebrates) in the blood. Respiration occurs by gills, lung, mantle, or body surface. Molluscs have kidneys, opening into the pericardial cavity and usually emptying into the mantle cavity. There is a nervous system of paired ganglia; sensory organs vary strongly between classes, but organs of touch, smell, taste, equilibrium, and vision all occur in some groups. Mollusca reproduce sexually; sexes are most commonly separate (with exceptions in the snails). The molluscan shells, which appear to be so very different in shape between the different classes, can (in fact) be mathematically constructed using a very simple formula for a spiral (look it up at this web site for a simple view, or go here for a real look into its mathematics).

• Class Gastropoda (snails). The gastropods are by far the most numerous and diverse of the molluscs, with about 40,000 living and 15,000 fossil species. In the present world about 5 out of 6 molluscs is a snail. They are usually sluggish, sedentary animals with heavy shells. The shell is always one piece, coiled or uncoiled, and is not subdivided into chambers. Live in the oceans, fresh water, and on land. Examples: snails, limpets, slugs, whelks, sea slugs, abalone.
• Class Bivalvia (clams and such). These bivalved molluscs (13,000 recent species) are mostly sedentary filter feeders that depend on ciliary currents produced by the gills to bring in food. Unlike the They have no head and no radula. Examples: mussels, clams, scallops, oysters, shipworms. Live in salt, brackish and fresh waters.
• Class Cephalopoda (there is a great web page on these: The Cephalopod pages). These are the most complex class of molluscs, all marine, active predators. The foot is modified and concentrated in the head region, and is funnel-shaped, expelling water from the mantle cavity (jet propulsion). The front part of the head is drawn out into a circle or crown of tentacles. Recent forms have no shells or an internal shell (cuttlebone), with the exception of the chambered Nautilus with a spiral, chambered shell. Extinct Ammonites also had a spiral (or modified), chambered shell. Examples: squids, octopi, nautiluses, cuttlefish, ammonites.
• Class Polyplacophora (chitons). These are somewhat flattened and have a convex dorsal surface that bears eight articulating plates, or valves.

Information on the Phylum.

"The staggering losses occasionally inflicted by the billions of locusts in Africa serve as only one reminder of our ceaseless struggle with the dominant group of animals on earth today: the insects. With nearly 1 million species recorded, and probably as many yet remaining to be classified, insects far outnumber all the other species of animals in the world combined. Numbers of individuals are equally enormous. Some scientists have estimated that there are 200 million insects for every single human alive today!" Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Pages 544-545.

Arthropods have a bilaterally symmetric, segmented body, with segments fused to various degrees in head, thorax, and abdomen; or cephalothorax and abdomen. The appendages (legs, gills, antennae) are jointed. In primitive forms there was one (double) pair to each segment (also called somite); later the number is often reduced, and appendages are modified or specialized (see below). Skeleton on the outside of the body (exoskeleton) contains protein, lipid, chitin, and often calcium carbonate, and shed (molted) at intervals. The muscular system is complex, and muscles are attached to the exoskeleton. There are striated muscles for rapid action; smooth muscles for visceral organs. The coelom is reduced, and most of the body cavity consists of sinuses, or spaces, in the tissues filled with blood. Complete digestive system; mouthparts modified from appendages. The circulatory system is open, with dorsal contractile heart, and arteries. Respiration by body surface, gills, tracheae (air tubes), or book lungs; there is an excretory systems with segmented kidney-like organs. The nervous system is well developed; dorsal brain connected by a ring around the gullet to a double nerve chain of ventral ganglia; well-developed sensory organs. Sexes usually separate, with paired reproductive organs and ducts; usually internal fertilization; often with metamorphosis; parthenogenesis in some forms.

There are many subphyla and classes. Present-day arthropods show great variety in morphology, but this variety can be reduced to a few consistent patterns. The basic pattern of making an arthropod involves the construction of a body from repeated segments. The key to the subdivision of all arthropods and their evolution over time is two fold: we look at the pattern of fusion and differentiation of segments (several segments may be fused to form a head, a middle, and a rear part), and at the specialization of appendages (such as legs, mouth parts, antennae). In what we think of as the most primitive (theoretical) arthropod, all segments are identical, and each segment bears the same type of double appendage. During evolution, segments fuse and appendages become specialized, depending upon their place in the animal. The arthropod appendage is usually called a leg, but it has several functions that we do not associate with legs in animals that are more similar to us (vertebrates). In primitive arthropods, the leg was a structure with two 'branches', called biramous (having two branches). One branch is used to walk, or to hand food towards the mouth, or to break up food in pieces (the function of jaws), or to 'feel' objects (antennae), and the other branch functions as a gill, taking up oxygen from the water. In what we consider more evolved arthropods the leg structure looses one of its branches, or branches are modified into jaws, antennae, pinchers, or wings (the latter formed from gill branches of the leg according to a recent study). Recent Arthropods are thus divided into four groups, depending upon the segments, how they are fused, and what appendages they bear:

• Uniramia: insects, millipedes, centipedes. In this group all appendages are built the from leg branch, the gill branches are lost, and breathing occurs through thin tubes into the body (trachea).
• Chelicerata: spiders, mites, scorpions, horseshoe crabs, extinct eurypterids. In this group all appendages on the front part of the body (jaws, legs) are formed from the leg branch; on the rear part (e.g., 'lung books' in spiders) they formed from the gill branch.
• Crustacea: crabs, lobsters, shrimp, barnacles, copepods, ostracods, isopods (pill bug). This group has five pairs of appendages on the head; the first two pairs have only one branch, and lie before the mouth. The last three pairs are behind the mouth, and are used for feeding. The appendages further back on the body are commonly double.
• Trilobita: extinct marine arthropods, which had one pair of appendages (antennae) before the mouth, and three after mouth on the head; the appendages on the body were double.

Information on the Phylum; the professional homepage for echinodermata researchers at the Calfiornia Academy of Sciences.

"Despite the adaptive value of bilaterality for free-moving animals, and the merits of radial symmetry for sessile animals, echinoderms confounded the rules by becoming free-moving but radial. That they evolved from a bilateral ancestor there can be no doubt, for their larvae are bilateral. They undergo a bizarre metamorphosis to a radial adult in which there is a 90^o reorientation in body axis, with a new mouth arising on the left side and a new anus appearing on the right side. [...] This constellation of characteristics is unique in the animal kingdom. It has both defined and limited the evolutionary potential of the echinoderms." Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Pages 604-605.

Echinodermata were very common constituents of Paleozoic faunas after the Cambrian period, from about 510 through 250Ma. In this period diversity had increased, suspension feeders that live on the sediment were common in addition to detritivores and carnivores.

All echinodermata have:

• An internal skeleton of calcium carbonate (limestone), consisting of ossicles (plates) with meshwork structure; covered by an epidermis
• 5-sided symmetry at least during part of their lives
• A rather complex hydraulic system used to move or catch prey with, called the water vascular system, which is of coelomic origin. It extends from the body surface as a series of tentacle-like projections (podia or tube feet).

Some groups have developed bilateral symmetry at least during parts of their lives; others remain 5-sided symmetric. Some forms use their water vascular system exclusively to move around with, other use muscles (sea cucumbers, brittle stars). Several forms of star fish can extrude their stomach through their mouth, and digest their food partially outside their body. There is no head or brain; few specialized sensory organs. The nervous system has a ring around the mouth and radial nerves. Locomotion is by the tube feet, which project from the ambulacral areas, or by movement of spines, or by movement of arms, which project from central disc of body. Digestive system usually complete; anus absent in sea cucumbers (see below). Respiration variable, in many cases by the tube feet. Excretory organs are absent. Sexes separate (except a few hermaphroditic); fertilization usually external. Development through free-swimming, bilateral, larval stages (some with direct development); metamorphosis to radial adult form.

Traditionally, Echinodermata have been subdivided into 5 groups:

• Sea urchins (Echinoidea). Echinoids have a compact body encased in test, or shell. The ossicles are modified to be close-fitting plates and make up the test. Echinoids have no arms, but their pentaradial structure is seen in the ambulacral areas, appearing as grooves on the test, ending at the area around the anus. "Regular" sea urchins are hemispherical with medium to long spines. They move by means of their tube feet, with some assistance from their spines. "Irregular" urchins, the sand dollars, have become bilaterally symmetric: their spines are usually very short and their bodies are flattened. They move chiefly by their spines.
• Starfish (Asteroidea). Sea stars typically have five arms (though they may have more) which merge gradually with the central disc.
• Brittle Stars (Ophiuroidea). Brittle stars differ from the sea stars in many ways. They also typically have five arms, but these arms are slender and distinctly set off from the central disc.
• Sea Cucumbers (Holothuroidea). Sea cucumbers are greatly elongated along the oral-aboral axis compared to the other echinoderms, and the ossicles are much reduced in most, so the animals are soft-bodied.
• Sea Lilies and Feather Stars (Crinoidea) with many extinct forms. In crinoids, the body disc has a leathery skin containing calcareous plates. Five flexible arms branch to form many more arms, each with many lateral pinnules arranged like barbs on a feather. Their ambulacral grooves are open and ciliated, and serve to catch small organisms and carry them to the mouth.

Information on the Phylum.

"The four distinctive characteristics that, taken together, set chordates apart form all other phyla are the notochord; single, dorsal, tubular nerve cord; pharyngeal pouches; and post-anal propulsive tail. These characteristics are always found at some embryonic stage, although they may be altered or may disappear altogether in later stages of the life cycle."

Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Page 637.

Chordates derive their name the notochord , a semi-flexible rod running along the length of the animal. In those chordates which lack bone, muscles work against the notochord to move the animal. All chordates have a notochord at some stage in their lives, but in some (such as tunicates) the notochord is lost in the adult. In others (such as the vertebrates) the notochord is present in the embryo, but in later stages is largely replaced and surrounded by the vertebrae, or backbones. The notochord runs beneath the dorsal nerve cord, which is another chordate feature. This is in contrast to protostomate animals such as annelids and arthropods, in which the main nerve cord is ventral. The chordate nerve cord is hollow, with pairs of nerves branching from it at intervals and running to the muscles. The forward end of the nerve cord is often enlarged into a brain. Pharyngeal slits are another chordate feature; these are openings between the pharynx, or throat, and the outside. They have been modified extensively in the course of evolution. In primitive chordates, these slits are used to filter food particles from the water. In fishes and some amphibians, the slits bear gills and are used for gas exchange. In most land living chordates, the "gill slits" are present only in embryonic stages; you had pharyngeal slits at one time. The slits are supported by gill arches, which have also been highly modified in various groups of vertebrates. Lastly, all chordates have a post-anal tail, or extension of the notochord and nerve cord past the anus. This feature is also lost in the adult stages of many chordates. The body shows bilateral symmetry, and has segmented muscles in a non-segmented trunk, with ectodermal, mesodermal and endodermal cells; well-developed coelom. The heart is ventral heart, with dorsal and ventral blood vessels; there is a closed blood system, a complete digestive system, and a cartilaginous or bony endoskeleton (interior skeleton) present in the majority of members (vertebrates).

Subphylum Hemichordata (alternatively placed in Phylum Hemichordata)

Information on Hemichordata.

About 100 species of soft-bodied marine animals, not widely known. There are two living groups, worm-like acron worms (Enteropneusts) and the vaguely Bryozoan looking Pterobranchs. They have gill slits (pharyngeal slits) like chordates, but have no notochord. The Pterobranchs live in colonies, with many tube-like individuals stuck together. The extinct graptolites (shapes like small hack saws; also see the UK graptolite page) are thought to represent similar colonies of small individuals.

Subphylum Vertebrata

• Superclass Agnatha ("Without jaws"): Those vertebrates without true jaws or appendages including the hagfishes and the lampreys.
• Superclass Gnathostomata ("Jawed mouth"): Those vertebrates with jaws and usually with paired appendages.
  • Class Chondrichthyes: cartilaginous fishes.
  • Class Osteichthyes: bony fishes.
  • Class Amphibia: amphibians.
  • Class Reptilia: reptiles.
  • Class Aves: birds.
  • Class Mammalia: mammals.