XVI] 



POLYMORPHISM 



379 



or in certain "cells" which contain rudimentary individuals. A free- 

 swimming larval form is usually found, which after a time comes to 

 rest and by budding forms a new colony. 



Just as in the colonies of Hydrozoa we found different 

 individuals set apart to perform different functions, so in Polyzoa 

 we find a similar specialisation. Certain individuals may be 

 modified to accommodate and protect the developing egg, but 

 perhaps the most remarkable modifications are the vibracula and 

 avicularia of the Cheilostome Polyzoa. The vibracula are long 

 hair -like processes which sweep 

 through the water ; the avicu- 

 larium consists of two snapping 

 jaws provided with powerful 

 muscles, likte the claws of a 

 lobster or the beak of a parrot 

 (Fig. 179). They are modifica- 

 tions of a "cell" and its oper- 

 culum. The avicularia occa- 

 sionally catch worms, Crustacea 

 and other animals whose pre- 

 sence might interfere with the 

 colony, and by their action they 

 probably prevent the larvae of Magnified. From Hincks. 



encrusting animals settling Oil &. Beak. c. Chamber representing 



the body-cavity of the modified in- 

 dividual, dm. Muscle which opens, 

 om. muscle which closes the man- 

 dible on the beak. md. Mandible, 



FIG. 179. An avicularium of Bugula. 



the operculum of the modified cell. 

 p. Stalk. 



the Polyzoan colony. They thus 

 serve somewhat the same pur- 

 pose as the pedicellaria of Ech- 

 inodermata, although they are 

 widely different in structure. 



Besides the sexual method of reproduction just mentioned 

 certain internal buds termed statoblasts are formed in the group 

 Phylactolaemata. Masses of cells arise from the funicle and become 

 enclosed between two watchglass-shaped chitinous shells whose 

 edges are kept together by a special ring of cells. As a rule, the 

 Phylactolaemata die down during the winter, but the statoblasts 

 persist and when spring recurs give rise to new colonies. A some- 

 what analogous process ensures the perpetuation of the species in 

 certain fresh-water Ctenostomata. 



Polyzoa are widely distributed throughout the sea, many occur- 

 ring in shallow water, but others have been dredged at great 

 depths. The Phylactolaemata and a few genera from other 



