ALIMENTARY CAXAL. 



55 



vascular canals). In the larger Polyps (Anthozoa) a tube derived 

 from an invagination of the oral disc projects into the central part 

 of the digestive cavity. This is known as the stomach of the polyp, 

 although it serves entirely for the introduction of food, and should 

 be called rather the buccal or O3sophageal tube (fig. 43). 



Organs for the prehension of food are found even with this simple 

 digestive system. For near the mouth are placed radially or bilate- 

 rally arranged appendages or processes of the body, which set up 



RK 



FIG. 44. Aurelia aurita seen from the oral surface. MA, the four oral tentacles with the 

 mouth in the centre ; Gk, genital folds; GH, opening of the genital pouches ; Rk, mar- 

 ginal bodies ; KG, radial canals ; T, tentacles at the margin of the disc. 



currents to convey small particles of food, or as tentacles seize foreign 

 bodies and convey them to the mouth (Polyps, Medusae) (fig. 44). 



Such appendages serving for the capture of prey may also be 

 placed further from the mouth (tentacles of Medusa, Siphonophora, 

 Ctenophora). 



When the digestive cavity acquires a wall distinct from the body 

 wall, and usually separated from the latter by the body cavity (ex- 

 cepting the parenchymatous worms), it appears in the simplest cases 

 as a blind tube, which may be either simple, bifurcated, or branched 



