120 



ZOOLOGY. 



with, its attached crown of tentacles, as well as the anterior 

 part of the membranous sac, within the posterior part 

 of this sac, by a process of inversion, 

 somewhat similar to that by which a 

 snail withdraws its eye-stalks. 



The mouth leads to a gullet and stomach, 

 a muscular gizzard sometimes intervening. 

 The intestine is bent upon itself, so that the 

 anal opening is placed near the mouth. The 

 whole peri visceral cavity is filled with fluid. 

 174. Circulation. No heart has yet 

 been discovered in any Polyzoon. The 

 products of digestion pass through the 

 walls of the intestine, and become mixed 

 with the surrounding fluid. 

 a, branchial iyg Respiration appears to be effected 



plume; w, ccsopha- r 



gus; c, stomach; by the ciliated tentacles, probably assisted 

 fnusf/fe^sui by the fluid which fills the peiivisceral 

 pended Win the cavity. This fluid is slowly replaced by 

 the action of the cilia which line the in- 

 terior walls of the endocyst. 



176. Nervous System. Between the mouth and the 

 anal aperture there is placed a single ganglion, which 

 sends off nerves in different directions. There is also 

 said to be a " colonial nervous system," forming a means 

 of communication between the various polypides in a 

 colony. It is believed to be by means of this nervous 

 system that the avicularia are able to continue their 

 motions after the death of the zooids, 



177. Development. In the Polyzoa the sexes are united 

 in the same individual. The reproductive organs are 

 attached to the inner surface of the endocyst. The ova 

 pass into the perivisceral cavity, where they are fertilized. 

 How they make their escape from the cell has not been 

 ascertained. The embryo is at first able to swim about 

 by the action of its cilia. After a time it attaches itself 

 to some fixed object, and originates a new colony, the 

 number of zooids being increased by continuous budding. 



FIG. 51. PLUM A' 



TELLA. 



