248 



CaiLENTERATA. 



;ixis and an outer layer of ectoderm. The entoderm forms an 

 otolith sac at the tip, while the ectoderm furnishes a nervous cushion 

 of ganglion cells and fibres and usually a simple e)'e spot. Less 

 evident is the effect of the lobing on the internal organs. The 

 gastrovascular system begins with a quadrate or X-shai:)ed mouth 

 (fig. 193). The perradial angles of the mouth are usually produced 

 into long curtain-like oral tentacles of great use in the capture of 

 food. The ' stomach,' \yhich begins just inside the mouth, gives ofE 

 four interradial (i.e., alternating with the corners of the mouth) 



Fig. 19.5.— Sense organs of Aurelia aurita. (After Schewiakoff.) cc, ectoderm; en, 

 entoderm; qv, gastrovascular space ; m, supporting layer ; n, cup eye ; nc, pigment 

 eye ; t)t, otolith sac ; ify, olfactory groove. 



pouches, the gastrogenital pockets. The epitlielium of these 

 pouches produces on the one hand a group of small gastral tentacles 

 (phacellas), each extremely mobile and supported bv an axis of 

 mesoglcea; on the other jilaited folds of the gonads, these being, in 

 contrast to the Ilydrozoa, of entodermal origin. In this the 

 Scyphomeduspe show relationships to the Authozoa. From the 

 central digestive sac arise the peripheral portions. These consist 

 in the larval medusie (Ephyra stage, fig. 192) of eight radial canals 

 to the sensory pedicels, and in most adult medusa? of these same 

 pouches and eight others, adradial in position, to the tentacles. In 

 some this primitive arrangement is complicated by an extensive 

 network of tubes (fig. 193). 



In the species \\ith an alternation of generations the egg pro- 

 duces a ciliated larva (fig. 19(1) which attaches itself and develops 

 into a scyphostoma. This scyphostoma is alwa^'s capable of ter- 



