246 



CWLESTERATA. 



rule; it is closely similar, even in the most different si^ecies, and 

 can even be lost (Pelagia), while the medusae are always well devel- 

 oped and present great variety of form. 



The scyphostoma (figs. 100, 11)1) recalls saioerficially Hydra, 

 but is distinguished externally by a small j)erisarcal cup in which 

 the aboral end is placed. Internally there are four longitudinal 

 folds i^rojecting into the gastral cavity and extending from the 

 margin of the mouth to the opposite pole. These septa or fceiiioke 

 appear in cross-section as small folds of entoderm supported by a 

 process of the supporting layer. They are important morpholog- 

 ically, since in budding they jiroduce the gastral tentacles ( p/iacelhe) 

 of the medusas. Further, they are the first api^earance of the septal 

 system, so strongly developed in the Anthozoa. 



The acraspedote medusa are large forms (four inches to four 



feet or more in diameter) with an 

 arched umbrella often of almost 

 cartilaginous consistency. They 

 are distinguished from the crasjje- 

 dotes externally by notches in the 

 margin of the umbrella, dividing 

 •cx^J the periphery into lobes. In the 

 common forms at least eight lobes 

 occur (figs. 192, 193), each notched 

 at its tip, and in the notch the 

 sensory pedicels bearing both ears 

 and eyes and covered by a lappet. 



Fig. 192.-Ephyra of Cotylorhiza. In SOme (flsf. 193, /. //) the Seu- 

 ( After Glaus.) (//, frastral tentacles , , „ t, ' ' / , 



(phacellas); rk, marginal (sensory) SOry lODCS lOllOW Cach Other, DUt m 



others the intermediate region is 

 also notched, the sensory pedicels then being found only on careful 

 searcli (fig. lOi). Tentacles, when present, spring from the 

 notches of the intermediate region. 



The sensory pedicels predicate the position of eight principal 

 radii, of which four are called the perrailii, the four alternating 

 with them the interradii. Adradii are radii lying betA\'een the 

 principal radii. 



Tlie lobing of the umbrella infiucnees all other structures. 

 There is no velum (hence these arc called Acraspedia), its place 

 being taken by a thick muscular mass (fig. 8(1, m) on the sub- 

 umbrellar surface. Instead of a nerve ring there are eight nerve 

 centres connected with the sensory pedicels. Each of these 

 pedicels (fig. 195) is a modified tentacle with an entodermal 



