THE FOLYZOA. 



181 



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. 



Pig. 135. — ^Branching marine Polyzoon. C'orm of Myrioztmm mbgracile^ 

 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 ai'e rarely 

 single, but a cormus, polyzoarinin or 

 polyzoon-stock is formed by the bud- 

 ding of numerous cells from the one 

 first formed. The single polyzoon is 

 called a polypide, and its cell a cystid. 

 In Pedicellina, the simplest polyzoon, 

 tlie polypide has no cystid or cell. 

 The cells are, in the mai-ine forms, 

 usually closed, and independent of 

 each other. The wall forming the 

 cell is called the endocyst ; it com- 

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

 the mesoderm forming the soft lining of the cell. 



Pig. \X.—HaloiMla iorealis, 

 enlarged. 



