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. Corm of Myriozoum subgracile, 
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. ‘he 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 polypide, and its cell a cystid. 
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 Fie. ae borealis 
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. 
