. Principles of modern biology. Biology. 644 - Heredity and Evolution . TENTACLE OF L0PH0PH0RE. AVICULARIUM COELOM Fig. 32-20. A bryozoan (Bugu/a); part of colony, showing structure of one individual animal. Note the avicularium, a pincerlike structure that serves to clip away debris and encrusting organisms from the surface of the colony. In fossilized Bryozoa, the avicularia (which represent highly modified individuals in the colony) tend to be very well preserved. Typically, each animal attaches itself to the substratum by means of a fleshy muscular stalk called the peduncle (Fig. 32-21). M
. Principles of modern biology. Biology. 644 - Heredity and Evolution . TENTACLE OF L0PH0PH0RE. AVICULARIUM COELOM Fig. 32-20. A bryozoan (Bugu/a); part of colony, showing structure of one individual animal. Note the avicularium, a pincerlike structure that serves to clip away debris and encrusting organisms from the surface of the colony. In fossilized Bryozoa, the avicularia (which represent highly modified individuals in the colony) tend to be very well preserved. Typically, each animal attaches itself to the substratum by means of a fleshy muscular stalk called the peduncle (Fig. 32-21). Much of the space within the shells (Fig. 32-21) is occupied by a coiled many-tentacled lophophore. This can be extended forth an- teriorly through the gap between the valves, when feeding is in progress. The body proper, which occupies a posterior position within the shells (Fig. 32-21), displays a bilateral symmetry, a well-developed mesoderm, a complete (usually tubular digestive tract, and a frilly lined coelomic cavity. Also there is a primitive hollow muscular heart, which maintains an irregular (lowing of the coe- lomic fluid, even though no blood vessels are present. Two to four primitive nephridia (p. 311) are present as excretory organs, but no specialized respiratory organs have been developed. Fossilized brachiopod shells (Fig. 32-22) are prevalent in marine rock strata extending back into the Cambrian period; and the record shows that the modern surviving kinds of Brachiopoda are relatively few in compari- son with the number of species that have be- come extinct. One modern genus (Lingula) represents perhaps the oldest of surviving multicellular animal groups. This animal has changed very little indeed since it first ap- peared in the Ordovician period more than 450 million years ago. The Nemathelminthes, or Nematodes. An amazing number and variety of these elongate cylindrical animals (Fig. 32-23) are found in various fresh-water bodies and in rich moist soils. Am
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