II. SCYPHOZOA 



221 



A knowledge of the development is necessary in order to understand 

 the medusa. The young medusa (Ephyra stage, fig. 185) is eight lobcd, 

 each lobe with a sensory pedicel in a notch at the tip. These lobes 

 indicate eight radii, the four passing through the angles of the mouth 

 being the perradii, the others the interradii, the adradii being between 

 the lobes (fig. 186). In most species the adradial regions increase with 

 growth, and at last form a circular margin to the bell, divided by the 

 notches of the original lobes (fig. 187), tentacles occurring only in the 

 adradial regions. The medusae differ 

 externally from those of the Hydrozoa 

 in the absence of a velum (hence acra- 

 spedote). 



Instead of a nerve ring there are 

 eight nerve centres connected with the 

 sensory pedicels. Each pedicel is a 

 modified tentacle (fig. 177, 7) its en- 

 todermal axis furnishing a statolith at 

 the end, and usually a simple eyespot. 



The gastrovascular system begins 

 with a quadrate or X-shaped mouth 

 (fig. 1 86). The angles of the mouth 

 are usually produced into long curtain- 

 like oral tentacles of use in the capture 

 of food. The 'stomach,' which begins just inside the mouth, gives off 

 four interradial pouches, the gastro genital pockets, each containing a 

 group of small gastral tentacles (phacelhc}, and the plaited folds of the 

 gonads, these being, in contrast to the Hydrozoa, of entodermal origin. 

 In this the Scyphomedusoe show relationships to the Anthozoa. From 

 the central digestive sac arise in the Ephyra stage (fig. 185) eight 

 radial canals to the sensory pedicels, and most adult medusa? have these 

 same pouches and eight others, adradial in position. In some this 

 primitive arrangement is complicated by a network of tubes (fig. 186). 



In the species with an alternation of generations the egg produces a 

 ciliated larva, the planula (fig. iSS) which attaches itself and develops 

 into a scyphostoma. This scyphostoma is capable of terminal, and often 

 of lateral, budding. The lateral buds always produce new scyphostoma', 

 the terminal, medusa?. In the latter the scyphostoma develops into a 

 strobila, becoming divided by circular constrictions into a series of saucer- 

 like discs, the young jelly-fish. As the successive discs become ready 

 they separate from the pile and swim away as epliyrcc. At lirst the 

 ephyra^ (fig. 185) have only four gastral tentacles, parts of the gastral 



FIG. 185. Ephyra of Cotylorhi-,i 

 (after Claus). gt, gastral tentacles 

 (phacelke); rk, marginal (sensory) 

 body. 



