Animal Classification

Animals form layers at the early embryo stage. In triploblastic bilaterians, the initial cells form the blastula, which is a single layer of cells in the shape of a sphere with an inner fluid called the blastocoel. Then, the sphere folds inward and forms an opening at one end, enclosing the blastocoel (yolk) between the layers (Figure 8‑7). There are two main types of triploblastic bilaterian animals. The protostome (insects and worms) mouth forms in the blastula and the anus forms in a second opening while the deuterostome anus forms in the blastopore (vertebrates, other chordates, and others) forms in a second opening. The initial gastrulation pattern of cnidarians is like that of triploblastic bilaterians, but there is only an endoderm and ectoderm and no blastocoel or archentron. [1]

The central cavity in sea anemones became the central cavity and later the gut in bilaterians. Muscles formed in circular and longitudinal patterns in worms (Figure 8‑9), enabling the worm to move. Over time, animals evolved sense organs and a mouth at the head end and an anus at the tail end, with a nerve cord running the length of the animal to send information to muscles and from from end to end.

Acoelomates, such as the flatworm in Figure 8‑10, were probable early triploblastic bilaterians. They have a mouth at one end but do not have an anus at the other end. They do not have a coelom (no internal cavity around the digestive tube) so they are called acoelomates.

The next step in evolution of animals was the pseudocoelomates, which are so named because they have a body cavity, but it is not defined as a true coelem since the cavity is not surrounded on all sides by the mesoderm. They include the rotifers (Figure 8‑11) and the roundworms (nematodes). Rotifers are part of the base of the food chain. They eat algae and other microorganisms. They are very small and are comprised of approximately 1,000 cells. They are like little vacuum cleaners as they sweep algae up to about 10 microns in diameter into their mouths. They are still an important part of the food chain because they consume algae and are then consumed by larger animals.

Coelomates have a true body cavity, which means that the mesoderm surrounds all sides of the cavity. All moving animals that have a digestive system inside a cavity are coelomates. Scientists divide coelomates into protostomes and deuterostomes. Unlike protostomes, in which the mouth forms in the blastopore (Figure 8‑12), the anus in deuterostomes forms in the blastopore, and the mouth forms in a second opening. Protostome means “first mouth” while deuterostome means “second mouth.” Invertebrates do not have an internal skeleton. Another differentiating characteristic is that protostomes have spiral embryo cleavage, and the deuterostomes have radial embryo cleavage. The mollusks (clams), arthropods (insects), and annelids (worms) are protostomes. The deuterostomes include seemingly unrelated animal phyla: echinoderms (starfish and sea urchins), urochordates (sea squirts), hemichordates (acorn worms), and chordates (vertebrates). As with protostomes, most deuterostome phyla are invertebrates.