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. Among the (roughly) 12,000 species of 

 Nemathelminthes, most are free-living, but 

 there are also quite a few parasitic species. 

 Moreover, some of the parasitic nematodes 

 are very important from a medical point of 

 view. 



Typically the elongate nematode body 

 tapers at both ends, although usually the 

 posterior end is more sharply pointed (Fig. 

 32-23). Many roundworms are microscopic, 

 but a majority range between 1 and 10 mm 

 in length; and one species, the guinea worm, 

 which is a subcutaneous parasite of man, 

 may reach a length greater than a meter. The 

 free-living forms, which can easily be seen 

 when one examines almost any sample of 

 good soil under the microscope, display a 

 characteristic thrashing type of movement. 

 The worms keep coiling and uncoiling rap- 

 idly and continuously. 



Typically the Nemathelminthes are triplo- 

 blastic, bilaterally symmetrical, nonsegmented 

 animals, with a complete tubular digestive 

 tract (Fig. 32-23). The body cavity is a 

 relatively small unlined pseudocoel. No pro- 

 boscis or lophophore is present; and there 

 are no clearly differentiated respiratory or 

 circulatory organs. One or two nephridia 



