183 



ZOOLOGY. 



The mouth is situated on a disk {lophophore, Fig. 137, B), 

 bearing the tentacles, wliich are liollow i^rocesses of the 

 body-walls, comnmnicating witli the body-cavity, tire blood 

 flowing into them, there being aerated, while tliey are exter- 

 nally ciliated. They serve both to catch food and for respir- 

 ation as makeshift gills. Hyatt states that the tentacles are 

 used not only to catch the prey, but for a multitude of other 

 offices. They are each cajsable of in- 

 dependent motion, and may be twisted 

 or turned in any direction ; bending 

 inwards, they take up and discard 

 objectionable matter, or push down 

 into the stomach and clear the 

 ossophagus of food too small to be 

 acted upon by the parietal muscles. 

 They are also employed offensively in 

 striking an intrusive neighbor, and 

 their tactile power, sensitive to the 

 slightest unusual vibration in the 

 water, warns the polypide of the ap- 

 proach of danger. 



The digestive canal hangs free in 

 the body-cavity, only attached by the 

 mouth and anus to the walls of the 

 body. It consists of a pharynx, a 

 large stomach, and an intestine which 

 lies by the side of the pharynx, since 

 the canal has a simple deep dorsal 

 flexure, the vent being situated on 

 the dorsal or cardiac side, near the 

 mouth. Usually the stomach is tied 

 by a sort of ligament {funiculus) to 

 a point on the body-walls, near the 

 mouth. The nervous system is rep- 

 resented by a double ganglion form- 

 ing a single mass situated between the mouth and vent ; it 

 is highly contractile and changeable in form. There is no 

 heart nor any circulatory apparatus. The sexes are united 

 in a single polypide, and form cellular masses growing ou 



Fig. 127.— Organization of a 

 Polyzoou. A, Palud-ieella Eh- 

 renhergii. B, Plumatellafru- 

 ticoaa. bi\ tentacular branchise 

 of lophophore ; ffi, ossophagns ; 

 y, stomach ; r, intestine ; a, 

 anus ; i, cell ; x, posterior, x'^, 

 anterior cord, at the insertion 

 of which into the body the 

 generative products are devel- 

 oped : i, testes ; o, ovary ; m, 

 retractor muscles of the ante- 

 rior portion of the cell ; m?', 

 principal retractor muscle. — 

 After Allman. 



