246 CCELENTERATA. 



role; it is closely similar, even in the most different species, and 

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

 oped and present great variety of form. 



The scyphostoma (figs. 190, 191) recalls superficially Hydra, 

 ibut is distinguished externally by a small perisarcal cup in which 

 the aboral end is placed. Internally there are four longitudinal 

 folds projecting into the gastral cavity and extending from the 

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

 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 produce the gastral tentacles ( pliacellce) 

 of the medusae. Further, they are the first appearance of the septal 

 system, so strongly developed in the Anthozoa. 



The acraspedote medusae 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 craspe- 

 dotes externally by notches in the 

 margin of the umbrella, dividing 

 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. 



FTG. 192. Ephyra of Cotylorhiza. Ill SOHie (fig. 193, /, //) the Sen- 

 (After Glaus.) gt, gastral tentacles i i < 11 11- 



(phacellee); rfc, marginal (sensory) SOry lobes follow each other, but 111 



others the intermediate region is 



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

 search (fig. 194). 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 perradii, the four alternating 

 with them the interradii. Adradii are radii lying between the 

 principal radii. 



The lobing of the umbrella influences all other structures. 

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

 being taken by a thick muscular mass (fig. 86, 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 



