126 ZOOLOGY. 



clay, somewhat after the manner of a snail, but much 

 more slowly. Opposite the base is the disc, in the 

 centre of which the mouth is placed. The circum- 

 ference of the disc is surrounded by two rows of ten- 

 tacles, arranged alternately in concentric circles. There 

 are sometimes as many as two hundred of these tentacles. 

 They are hollow tubes, communicating with the cavity of 

 the body. The animal is able, by contracting its body 

 walls, to force water into these tentacles, and thus pro- 

 trude them to a considerable length. It has also the 

 power of withdrawing the water, and contracting them. 

 Sometimes the tubes are open at the tips also. These 

 tentacles are the prehensile organs of the animal. They 

 are furnished with those singular weapons called thread 

 cells. Each of these cells consists of a membranous sac, 

 containing a coiled up thread. When the cell is touched, 

 a filament, barbed at the point, and serrated along the 

 edges, is projected from it with sufficient force to pene- 

 trate the tissues of most animals with which it comes in 

 contact. These thread cells are also found in the lining 

 of the alimentary canal, and in the outer integument, 

 which is of a soft leathery structure. 



The mouth leads into a tubular stomach, which is not 

 followed by an intestine, but opens below into the general 

 cavity of the body by a wide aperture. The indigestible 

 portions of the food are ejected out of the mouth. The 

 alimentary tube terminates about half-way between the 

 mouth and the base. The intervening space, between 

 this tube and the body walls, is divided by a number of 

 vertical partitions, or mesenteries, into compartments. 

 Some of the partitions, which originate in the body wall, 

 do not reach the whole way to the outer lining of the 

 alimentary tube. These compartments communicate with 

 the open bases of the tentacles. To the walls of the 

 mesenteries the reproductive organs are attached. The 

 sea-anemones have generally the sexes in distinct in- 

 dividuals. The embryo is a minute oval body, and is able 

 to swim about freely by the action of its cilia. 



