THE POLYZOA. 



139 



The simpler form of Polyzoon is a worm-like creature 

 enclosed in a minute, deep, horny cell, with the alimentary 

 canal bent on itself and terminating in a vent situated near 

 the mouth, the latter surrounded, in the fresh-water forms, 



Fig. 95. Branching marine Polyzoon. Corru of Myriozouni ti/bgracile, 



natural size. 



with a horseshoe-shaped crown, or in the marine species a 

 circle of slender ciliated tentacles. The animal when dis- 

 turbed withdraws into its tube or shell, which is often trans- 

 parent, allowing it to be examined 

 when alive. The cells are rarely 

 single, but a cormus, polyzoarium or 

 polyzoon-stock is formed by the bud- 

 ding of numerous cells from the one 

 first formed. The single polyzoon is 

 called a polyp ide, and its cell a cyst id. 

 In Pedicellina, the simplest polyzoon, 

 the polypide has no cystid or cell. 

 The cells are, in the marine forms, 

 usually closed, and independent of 

 each other. The wall forming the 

 cell is called the endocyxt ; it com- 

 prises the ectoderm proper, with a portion (parietal layer) oif 

 the mesoderm forming the soft lining of the cell. 



Fig. 96 . Halopfrila borealis, 

 enlarged. 



