Major Groups of Multicellular Animals

The simplest metazoa are tiny symbiotic worms of the phylum (or subkingdom) Mesozoa, which live in the kidneys of deep sea-dwelling cephalopods (octopi and squid). Each worm is made up of only 25 cells in a single layer enclosing one or a small number of elongated axial cells. Mesozoa have been regarded as parasitic, but they appear to facilitate excretion of NH3 by the host through acidification of the urine. Porifera or sponges are the most primitive of multicelled animals. They lack distinct tissues but contain several specialized types of cells. The body is formed by stationary cells that pump water through the pores to bring food to the sponge. Within the body amebocytes work in groups to form the spicules of
calcium carbonate, silicon dioxide, or the protein spongin. Sponges appear to lack a nervous
system. Individuals of the next most complex major phylum, Cnidaria (formerly oelenterata), are radially symmetric with two distinct cell layers, the endoderm and ectoderm. Many species exist both as a polyp or hydra form and as a medusa or jellyfish. The jellyfish apparently has no brain but the ways in which its neurons interconnect in a primitive radial net are of interest. The Cnidaria have a very simple body form with remarkable regenerative powers. The freshwater hydra, a creature about 1 cm long, contains a total of ~105 cells. A complete hydra can be regenerated from a small piece of tissue if the latter contains some of both the inner and the outer cell layers. The body of flatworms (phylum Platyhelminthes) consists of two external cell layers (endoderm and ectoderm) with a third layer between. A distinct excretory system is present. In addition to a nerve net resembling that of the Cnidaria, there are a cerebral ganglion and distinct eyes. One large group of flatworms, the planarians (typically about 15 mm in length, inhabit freshwater streams. They are said to be the simplest creatures in which behavior can be studied.

Some lower forms of Metazoa. (A) Mesozoa (25 cells). After C. P. Hickman. (B) A small asconoid sponge. After C. A. Villee, W. F. Walker, Jr., and R. D. Barnes. (C) Ameboid cells of a sponge forming spicules. After Hickman.

Many parasitic flatworms (tapeworms and flukes) attack higher organisms. Among them are the Schistosoma, tiny worms that are transmitted to humans through snails and which attack the blood vessels. The resulting schistosomiasis is one of the most widespread debilitating diseases on earth today, affecting 200 million people or more. The roundworms (Nematoda) have, in addition to the enteron (alimentary tract), a separate body cavity. Free-living nematodes abound in water and soil but many species are parasitic. They do enormous damage to plants and to some animal species. Trichina, hookworms129a, and filaria worms attack humans. However, in the laboratory the 1-mm-long, 810-cell nematode, Caenorhabditis elegans has become an important animal. In 1963 Sydney Brenner launched what has become a worldwide effort to make this tiny worm the equivalent in the animal kingdom of . coli in the bacterial world. The 108 nucleotides in the worm’s six chromosomes contain ~13,600 genes. C. elegans has become an important animal in which to study differentiation. Already the exact lineage of every cell has been traced, as has every connection among the 302 neurons in the animal’s nervous system. The elated rotifers, 130 with whirling“wheels” of cilia on their heads and transparent bodies, are a delight to the microscopist. Like nematodes, they are “cell constant” organisms. The total number of cells in the body is constant as is that in almost every part of every organ. Part of the develop mental plan of such organisms is a “programmed cell death”.

(A) Hydra. After Loomis. (B) The medusa stage of Obelia, a hydroid coelenterate.

The Annelida (segmented worms) are believed to be evolutionary antecedents of the arthropods. Present-day members include earthworms, leeches, and ~105 species of marine polychaetes. Annelids have a true body cavity separate from the alimentary canal and lined by a peritoneum. They have a welldeveloped circulatory system and their blood usually contains a type of hemoglobin. About 106 species of arthropods (80% of all known animals) have been described. Most are very small. These creatures, which have a segmented exoskeleton of chitin and other materials, include the horseshoe crabs, the Arachnida (scorpions, spiders, and mites), the Crustacea, Myriopoda (centipedes and millipedes), and the Insecta. Important biochemical problems are associated with the development and use of insecticides and with our understanding of the metamorphosis that occurs during the growth of arthropods. The fruit fly Drosophila melanogaster has provided much of our basic knowledge of genetics and continues to be the major species in which development is studied. Among the molluscs (phylum Mollusca) the squids and octopuses have generated the most interest among biochemists. The neurons of squid contain giant axons, the study of which has led to much of our knowledge of nerve conduction. Octopuses show signs of intelligence not observed in other invertebrates whose nervous reactions seem to be entirely “preprogrammed.” The brains of some snails contain only neurons, some of which are unusually large. The Echinodermata or spiny-skinned animals (starfish, sea urchins, and sea cucumbers) are regarded as a highly advanced phylum. Their embryological development has been studied intensively. The phylum Chordata, to which we ourselves belong, includes not only the vertebrates but also more primitive marine animals that have a spinal cord. Among these primitive species, which may be related to early ancestral forms, are the tunicates or sea squirts. They have a very high concentration of vanadium in their blood.

(A) A planarian, length 15 mm. After Hickman. Diagram of digestive and nervous sytems;cutaway section shows ventral mouth. Small drawing shows pharynx extended through ventral mouth. (B) The nematode Caenorhabditis elegans. Ascaris is very similar inappearance. From Buchsbaum. (C) A rotifer, Philodina (~103 cells). After C. A. Villee et al.