Dividing the Protostomes Adult bilateria share a similar body plan that many people refer to as a "tube within a tube," a plan emerging from the three germ layers. The ectoderm develops into the body wall, the outer tube. The endoderm develops into an inner tube, or gut, with two openings: the mouth and the anus. The mesoderm develops into a fluid-filled inner cavity between the ectoderm and the endoderm. Organs develop inside this cavity. The two major divisions of bilateria are based on a fundamental difference in the way this body plan emerges during embryonic development. The germ layers and gut form by the process of gastrulation. During gastrulation, a pore called a blastopore forms and develops into the archenteron, the opening that will develop into the gut. In protostomes, or "first-mouths," this pore eventually forms the mouth, whereas the anus forms later after development of the gut. In deuterostomes, or "second-mouths," this pore forms the anus and the mouth develops later. Other fundamental differences in protostome and deuterostome embryonic development occur in the patterns of cleavage divisions.Many protostome groups undergo spiral cleavage, in which the planes of cell division are diagonal to the vertical axis of the embryo.Radial cleavage, on the other hand, is characteristic of deuterostomes.The stage at which the fate of embryonic cells becomes determinate differs as well. In many protostome groups, a cell isolated at the four-cell stage cannot develop into a complete embryo (determinate cleavage). Most deuterostomes experience indeterminate cleavage until a later stage. Principles of Biology

Figure 2 Cleavage differences in protostomes and deuterostomes. Groupings based on possession of a coelom or pseudocoelom drawn from molecular evidence do not correlate with previously defined evolutionary relationships drawn from morphology. Principles of Biology

Figure 3 Classic vs. modern bilateria phylogeny. Protostomes Figure 3 Classic vs. modern bilateria phylogeny. Groupings based on possession of a coelom or pseudocoelom drawn from molecular evidence do not correlate with previously defined evolutionary relationships drawn from morphology. Principles of Biology

Members of phylum Rotifera have specialized organ systems. Protostomes Members of phylum Rotifera have specialized organ systems. Rotifers are remarkably tiny animals. They are smaller than many protists but are multicellular and actually possess miniscule organs.Rotifers were named for the wheel-like lophophores that draw a constant stream of water into the alimentary canal (a digestive tract with two openings: the mouth and anus). Rotifers can produce asexually by parthenogenesis, in which a female produces female offspring from unfertilized eggs.Molecular and fossil analyses indicate that the bdelloids have likely been all female for 100 million years. Principles of Biology

Figure 4 Photomicrograph of a rotifer. Protostomes Figure 4 Photomicrograph of a rotifer. The rotifer Brachionus calyciflorus is a miniscule predator of small plankton in aquatic ecosystems. Principles of Biology

Members of phylum Platyhelminthes have a central nervous system. Protostomes Members of phylum Platyhelminthes have a central nervous system. The Platyhelminthes, or flatworms, have thin bodies flattened along the dorsal-ventral axis.Flatworms lack a coelom and organs for circulation or gas exchange. All body cells are close to both the gut and the surrounding environment.Free-living flatworms such as planarians are important predators and scavengers.Parasitic flatworms cause diseases in host animals, including humans. Most tapeworms possess suckers and hooks on their anterior end that are adapted to attaching to the interior lining of a host's digestive system. They lack a mouth and gut, simply absorbing nutrients from the host. Principles of Biology

Figure 5 Marine flatworm. Protostomes Figure 5 Marine flatworm. The flatworm Pseudoceros ferrugineus is an inhabitant of coral reefs. Principles of Biology

Members of phylum Annelida have equal segments. Protostomes Members of phylum Annelida have equal segments. Annelida is distinguished by a division of the body into segments of roughly equal size.Segmentation represents an advance in complexity. The degree of complexity increases as the segments specialize and diverge morphologically.Asexual reproduction occurs by transverse fission and fragmentation. The body separates into segments along the transverse axis, and each segment can develop into a new individual.Sexually reproducing polychaetes typically have separate males and females, whereas oligochates and leeches are typically hermaphroditic Principles of Biology

Figure 6 Feather duster worms. Protostomes Figure 6 Feather duster worms. The feathery tentacles of these tube-dwelling worms are modified chaetae (bristles) at the anterior end of the worm. Most of the worms' bodies are hidden within the tubes they have constructed for protection from predators. Principles of Biology

Figure 7 Molluscan body plans. Protostomes Figure 7 Molluscan body plans. The phylum Mollusca is one of the most diverse animal phyla. Accordingly, its members exhibit a range of body plans. Most, but not all, mollusks have a shell that is secreted by mantle tissue. In the squid, the mantle tissue composes the outside surface of the body, and the shell is reduced to a small plate located under the mantle. Foot tissue is generally specialized for movement, although in bivalves (such as oysters), the viscera also extend into the foot. The radula is a hardened structure used for chewing that is absent in filter-feeding bivalves. Principles of Biology

Figure 8 Mollusc diversity. Protostomes Figure 8 Mollusc diversity. The Squid (Illex illecebrosus) — a cephalopod, Giant Clam (Tridacna sp.) — a bivalve, and Opalescent Sea Slug (Hermissenda crassicornis) — a gastropod, are examples of the diverse forms that emerge as variations on a single body plan. All possess the three basic characteristics of molluscs: a foot, a visceral mass, and a mantle. Principles of Biology