DISCOPHORA 323 



Order IV. Discophora. 



This order contains not only by far the greater number of 

 the species of Scyphozoa, but those of the largest size, and all 

 those that are familiar to the seaside visitor and the mariner 

 under the general term jelly-fish. 



They may be distinguished from the other Scyphozoa by 

 several well-marked characters. The umbrella is flattened and 

 disc-shaped or slightly domed, but not divided by a coronary 

 groove. The perradial angles of the mouth are prolonged into 

 long lips, which may remain free (Semaeostomata) or fuse to 

 form an elaborate proboscis (Ehizostomata). 



Sub-Order I. Semaeostomata. 



In this sub-order the mouth is a large aperture leading into 

 the cavity of the manubrium, and is guarded by four long grooved 

 and often tuberculated lips. The margin of the umbrella is 

 provided with long tentacles. 



Fam. 1. Pelagiidae. Semaeostomata with wide gastric 

 pouches, which are not united by a marginal ring sinus. Pelagia, 

 which forms the type of this family, has eight long marginal 

 tentacles. It develops directly from the egg, the fixed Scyphi- 

 stoma stage being eliminated. 1 It is probably in consequence 

 of this peculiarity of its development and independence of a 

 shore for fixation that Pelagia has become a common and wide- 

 spread inhabitant of the high seas. In the Atlantic and Indian 

 Oceans P. phosphor a occurs in swarms or in long narrow lines 

 many miles in length. It is remarkable for its power of emitting 

 phosphorescent light. In the Atlantic it extends from 50 N. 

 to 40 S., but is rare or absent from the colder regions. P.perla 

 is found occasionally on the west coast of Ireland. Chrysaora 

 differs from Pelagia in the larger number of tentacles. There 

 are, in all, 24 tentacles and 8 statorhabs, separated by 32 

 lobes of the margin of the umbrella. C. isosceles is occasionally 

 found off the British coast. It passes through a typical 

 Scyphistoma stage in development. Dactylometra, a very 



1 A stage in development before the formation of the sub-umbrellar cavity, hut 

 subsequent to the formation of the first tentacles, is regarded as homologous with 

 the Scyphistoma stage of other Scyphozoa. 



