428 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



islands. Most of the coral polyps have the structure of sea anemones, 

 which resemble the hydroid polyps, but differ from them in a number 

 of essential ways. 



THE BRYOZOA 



The bryozoans are another group of colonial invertebrates, known 

 also as Polyzoa. Most of them are branching colonies attached to 

 supports such as shells, rocks, and piles or other similar things along 

 the seashore. In one genus, Cristatella^ the colony has a matlike form 

 and freely creeps about. The individuals resemble the polyps of a 

 hydroid colony, each having a circle of tentacles around the mouth. 

 In structure, however, the bryozoans are of a higher type of organi- 

 zation than the coelenterates ; they are animals with a coelomic body 

 cavity between the body wall and the alimentary canal. Some spe- 

 cies, including the common Bugula avicularia, are noted for the fact 

 that most of the zooids are armed each with a small appendage resem- 

 bling a bird's head, which turns from side to side with active snap- 

 ping movements of the bill. This appendage, the avicularium, how- 

 ever, is a reduced zooid, showing that a differentiation has taken place 

 among the individuals of the colony. 



THE HEMICORDA 



Among the prevertebrate animals with a notocord instead of a back- 

 bone, some species of the Hemicorda, so named because the notocord 

 is restricted to the anterior part of the body, are colonial, but the 

 members are free individuals. Most of them live in tubes of their own 

 construction formed of a secretion from a lobe on the ventral side of 

 the head. A colony usually consists of groups of tubes more or less 

 cemented together. Each animal is anchored in its tube by a long 

 slender tail with an adhesive disk at the end, but it can leave the tube 

 and creep about on the outer surface. 



Of special interest are members of the colonial genus Cephalo- 

 disciis which, instead of constructing individual tubes, build a com- 

 mon house with a central chamber in which they all live together. 

 Througli openings in the wall, however, the animals can crawl out 

 and creep about on the outside. Species of C ephalodiscus are small 

 animals living fairly deep in the ocean. Each animal has an oval or 

 elongate body crowned with a group of tentacles, at the base of which 

 is a flattened head lobe on the ventral side. Both the mouth and the 

 anus open anteriorly. Posteriorly the body is prolonged into a long 

 slender taillike stalk with an adhesive disk at the end. A C ephalo- 

 discus colony is founded by a single sexually developed animal, which 

 reproduces asexually by budding. The buds, however, develop into 

 free individuals, which together build their common dwelling, an ir- 

 regular structure with spinelike prolongations. 



