Biological Diversity Classified

Kingdom Animalia

Unlike Protista, Fungi, and Plantae, there is little argument about what groups are placed in this kingdom. Animals are multi-cellular heterotrophs, whose cells have no cell wall and individuals of which are usually motile or have motile structures. Except for Porifera, animals generally have cells that clearly fit into one of the four basic tissue types: epithelium, connective, muscle and nerve.

Phylum Porifera

The sponges are characterized by having no tissues and a body of only two cell layers (diploblastic). Although there are three levels of complexity of sponge bodies, and sponge bodies are supported by a variety of materials, the basic body plan of a sponge is that a layer of flat cells is on the outside and a layer of flagella-bearing cells is on the inside, surrounding a hollow chamber, the spongocoel. The beating flagella move water through pore-shaped cells, into the spongocoel, and out an opening (osculum); this allows the sponge to filter-feed. An example is:


This slide shows cross sections of a sponge of intermediate complexity. You can see the radial canals (R) radiate outward from the spongocoel (S); incurrent canals (I) allow water in from the environment to the porocytes. With a good slide, you may see pointed spicules or recognize the two cell layers.

Phylum Cnidaria

The cnidarians, which include the hydroids, jellyfish, corals and sea anemones, are characterized by having two tissue layers (outer = epithelio-muscle layer; inner = gastrodermis); two body forms (polyp is a cylinder with dorsal tentacles around the mouth, and is normally sedentary; medusa is an umbrella-shaped, free-swimming form with marginal or ventral tentacles); a gastrovascular cavity (or incomplete digestive system = one opening); and, especially, stinging cells. Most are marine; many are colonial. Examples of various forms can be seen in:


This species typically occurs as a connected colony of individual polyps produced asexually by budding. Gastrozooids (F) possess tentacles (T) and are specialized for feeding; they also possess a gastrovascular cavity (G). Gonozooids (R) are specialized for reproduction (budding, which produces tiny medusae (M)). The free-living medusa form is the sexual stage; reproductive organs (O) are not visible in immature medusae, but are in mature medusae.

Phylum Platyhelminthes

This phylum contains the free-living flatworms (e.g., planarians) and the parasitic flukes and tapeworms. The most obvious identifying feature of this phylum is the flat, ribbon-like shape of these worms. They do not have a body cavity of any sort between their body wall and their internal organs. Most have a digestive system in the form of a gastrovascular cavity; tapeworms have no digestive system. Many species in this group of worms are parasitic; they are hermaphrodites. Examples are:


This is a fairly common aquatic scavenger; it has its mouth in the middle of its ventral surface. An injected specimen shows the extent of the gastrovascular cavity (G), which extends in all directions from the pharynx (P); eye spots (E) are also visible. A cross section of a planarian clearly shows the acoelomate condition, with no space between the body wall (B) and the internal organs; the epidermis (S) is also visible.


This tapeworm shows the body form typical of this group of parasites. The scolex (S) is at the "head" end and possesses hooks and suckers for attaching to the gut wall of its host. What appear to be segments (but are more like individual members of a colony) are termed proglottids (P). These do contain such structures as nerves and excretory organs, but are primarily packages of reproductive structures; mature proglottids are mostly filled with eggs (E).

Phylum Nematoda

This phylum contains the roundworms; there are free-living forms as well as parasites of both animals and plants. They have no readily seen unique characters beyond their smooth, round, worm-shaped body; internally, they have a pseudocoelom in which their organs float freely. They have a complete digestive system, the least complex group of animals to have one. There are separate males and females; females are larger than males. 


These are simple intestinal roundworms. In both the male and the female, one can see the body wall (B), various sections of the reproductive system (R), the digestive system (G), and the cuticle (C), which serves as a protective coating for the body. Males exhibit a sperm duct (S) with sperm, whereas females have a uterus (U) filled with eggs.

 Phylum Rotifera

These animals, too, are pseudocoelomate, and were once grouped in the same phylum as nematodes. Though the size of many protists, they are complex animals with several organ systems, including a complete digestive tract. They have a coronal ring of cilia at the anterior end, surrounding their mouths. 


At their posterior end, rotifers have a foot (F) which allows them to attach to the substrate for feeding. They possess a muscular grinding structure, the mastax (M), between their mouth and stomach (S); also visible are the ovary (O) and coronal cilia (C).

Phylum Annelida

These are the only group of segmented worms; the segmentation is externally obvious as a series of rings along the body. They have complete digestion, a closed circulatory system, a ventral nerve cord, a nephridial excretory system, and a true body cavity or eucoelom. An example is:


The earthworm is fairly typical of one class of annelids. A cross section of the worm allows a view of the basic structure of most of the body. The intestine (I) is most obvious, with the typhlosole (T) protruding into the lumen (inner space). Also visible are the dorsal artery (A) and the ventral nerve cord (N), above and below the intestine, respectively; the muscles of the body wall (B) are quite obvious.

Phylum Arthropoda

Arthropods are characterized by a segmented body covered by an exoskeleton and possessing jointed appendages. This is a very diverse group, and the most advanced group of protostomes. It contains several subphyla and classes.

Class Arachnida

These are members of the subphylum Chelicerata, so named for possessing fang-like chelicerae rather than mandibles (true jaws). Arachnids are distinguished by having four pairs of walking legs, although many have a pair of rather leg-like chelipeds which may be used in feeding or prey capture; they also have the head and thorax body regions fused as a cephalothorax. Examples are:


Only vaguely looking like scorpions, these have "pinchers" at the ends of their large chelipeds but do not possess the sting at the end of the abdomen, however. Note cephalothorax (T), abdomen (A), four pair of legs (L), chelipeds (P) and chelicerae (F). A close-up of the cheliped shows the "pinchers."

Class Insecta

Any arthropod with wings must be an insect. Except for a few immature arachnids, anything with six legs (hexapod) is an insect. This group is extremely diverse. Examples include ants and fleas.

Class Crustacea

Technically, crustaceans are distinguished by two pair of antennae. Practically, an abundance of legs or appearance like crayfish, shrimp or crabs is usually your best bet; they possess gills, though some are terrestrial. An example is Gammarus.