356 ^W£ ANIMAL KINGDOM 



out to each side into whorls. The cilia draw water toward the animal 

 and food particles are passed down a ciliated tract at the tentacle bases 

 to the mouth. In both phyla the eucoelom, circulatory system and meta- 

 nephridia are well developed, so that they are obviously eucoelomates. 

 Each group contains few living species. 



Phoronids live in membranous tubes in the sand or cemented to 

 rocks. They have a long body with a U-shaped gut, the anus opening 

 just behind the lophophore. During feeding, the lophophore is extended 

 from the tube into the open water. Brachiopods are encased in a bi- 

 valved shell, the ventral shell being slightly larger than the dorsal shell. 

 They have a superficial resemblance to the bivalved molluscs. In some 

 species a stalk projects through the hinge to attach the animal to rocks; 

 in others, the stalk is absent and the animals lie free on the bottom. 

 Either the anus opens to one side of the lophophore or the gut ends 

 blindly without an anus. During feeding the shells are opened slightly 

 and water is drawn in. 



Although the Brachiopoda are a minor phylum today, they were a 

 major group in the past. Throughout the Paleozoic era they were 

 abundant, with thousands of species in all the oceans of the world. 

 Most of the fossil shells that can be found today in shale and slate de- 

 posits are not those of clams, but of brachiopods. 



The relation of these phyla to other eucoelomates is obscure. Their 

 early development is variable, but in all cases shows a wide departure 

 from the spiral cleavage-trochophore pattern. Cleavage follows a sim- 

 pler pattern. Some species are schizocoelous while others are entero- 

 coelous. In some the mouth forms from the blastopore (characteristic of 

 the mollusc-arthropod series) while in others it is a new opening (char- 

 acteristic of the echinoderm-chordate series). In the light of these varia- 

 tions the two phyla are sometimes considered to represent survivors of 

 an intermediate group between the two major series, a group that pos- 

 sibly was involved in the evolution of the echinoderm-chordate series 

 from the "main line" with its spiral cleavage. 



162. The Bryozoa 



The Bryozoa are minute colonial animals (Fig. 18.4) that also have 

 a lophophore. They are common in both salt and fresh water. They have 

 a long fossil record, but apparently were never a dominant group. Al- 

 though they have no circulatory system or excretory organs, they have 

 a well developed eucoelom. The absence of some structures is probably 

 an adaptation to small size. The lophophore is circular or U-shaped, 

 and the cilia draw water toward the animal. Food particles are swirled 

 into the mouth. The tentacles bend actively and are somewhat selective, 

 knocking large debris to one side and sometimes hitting smaller particles 

 toward the mouth. The colonies are formed by asexual budding, and 

 often a particular individual in the colony will degenerate, to be re- 

 placed by the development of a surviving bud of tissue. 



The position of bryozoans in the animal kingdom is debatable. 

 They are usually grouped with the brachiopods and phoronids to form 



