ALIMENTARY CANAL OF CGELENTERATA. 



115 



Fig. 43. A Thauniantias. 

 A From the lower surface. 

 B Seen in section. In the 

 middle of the body is the 

 stomach, from which the 

 radial canals pass to the 

 circular canal. 



concave portion of the gelatinous disc, and consists of a stomach 

 placed in the middle of this cavity, and of hollow spaces which 

 proceed from it. The stomach either lies 

 directly beneath this surface, or is placed on 

 a special stalk, which arises from it, and is 

 often of considerable size. This free projec- 

 tion of an organ, which in other animals is 

 hidden within the body, is explained by the 

 differentiation of the stomach of Hydrome- 

 duste from the most anterior portion of the 

 body of the Hydroid-Polyps, so that it does 

 not represent a single organ, but a complete 

 portion of the body. The mouth is generally 

 surrounded by tentacular organs, or pointed 

 prolongations of the wall of the stomach ; 

 it seldom opens into a narrow portion resem- 

 bling an oesophagus. In most Hydromedusaa 

 the stomach is separated from the space that 

 lies behind it by a ring which is developed at 

 its base ; by the contraction of this ring, the 

 stomachal cavity can be shut off from the 

 rest of the gastro vascular system. The stomach varies greatly in 

 form and size. It projects far beyond the edge of the bell-shaped 

 umbrella in the SarsiadaB. The 

 hollow spaces which are distributed 

 in the sub-umbrella arise from the 

 base of the stomach, or from the 

 space which lies behind it, and have 

 the form of narrow canals, or of 

 wide pouch-like diverticula. The 

 narrow canals take a radial course 

 (Fig. 43) to the edge of the um- 

 brella ; they are simple or regularly 

 branched, and they open into the 

 circular canal, which often sends 

 processes into the marginal ten- 

 tacles also. On their way to the 

 margin, the radial canals may form 

 diverticula, which are functionally 

 connected with the generative 

 apparatus (cf. § 96). 



In theiEginidas and Discophora 

 the gastric cavity passes directly 

 into the radial enlargements; 

 these latter are derived from 

 simpler canals. Narrower canals 

 sometimes, indeed, alternate with 

 wider spaces. The canals are 

 branched (Fig. 44, gv), or form, as in 



the Rhizostomidaa, a peripheral network. As the gelatinous substance 



I 2 



Fig. 44. Aurelia aurita. Half of the 

 lower surface is seen, a Marginal bodies. 

 b Oral arms, v Gastric cavity, gv Canals 

 of the gastrovascular system, which 

 branch towards the edge and unite into 

 a circular canal, ov Ovaries. 



