I IO MANUAL OF ZOOLOGY. 



Fam. V. Rhizophysiadce. Hydrosoma without either nectocalyces or 

 hydrophyllia. Coenosarc filiform, Pneumatocyst small. 



Fam. VI. Physaliada, Pneumatocyst occupying almost the whole of the 

 thick and irregularly fusiform ccenosarc. No nectocalyces or hydro- 

 phyllia. 



Fam. VII. Velellidce. Hydrosoma without nectocalyces or hydrophyllia ; 

 with short, simple, or branched submarginal tentacles. A single central 

 principal polypite. Pneumatocyst flattened, divided into chambers by 

 numerous concentric partitions, and occupying almost the whole of the 

 discoidal ccenosarc. 



CHAPTER X. 



DISCOP HORA. 



SUB-CLASS III. DISCOPHORA (Acalepha * in part). Since the 

 sub-class contains only a single order, that of the Medusida, a 

 single definition necessarily suffices for both. The Medusidce 

 are defined as " Hydrozoa whose hydrosoma is free and oceanic, 

 consisting of a single nectocalyx, from the roof of which a single 

 polypite is suspended. The nectocalyx is furnished with a system 

 of canals. The reproductive organs are as processes either of the 

 sides of the polypite or of the nectocalycine canals " (Greene). 



The Medusidce comprise most of the organisms commonly 

 known as Jelly-fishes or Sea-nettles, the last name being de- 

 rived from the property which some of them possess of severely 

 stinging the hand, this power being due to the presence of 

 numerous thread-cells. As employed by modern naturalists, 

 the order is very much restricted, and it is by no means impro- 

 bable that it will ultimately be entirely done away with, very 

 many of its members having been shown to be really the free 

 generative buds of other Hydrozoa. As used here, it corre- 

 sponds to part of the Gymnophthalmate Medusce of Professor 

 E. Forbes, the Steganophthalmate Medusce of the same author 

 being now placed in the sub-class Lucemarida. 



* The old sub-class of the Acalephce contained the Gymnophthalmate 

 Medusa ( = the Discophora}, and the Steganophthalmate Medusa ( = the 

 Lucemarida in part), the two being placed in a single order under the 

 name of Pulmograda . The Acalepha also contained the Ctenophora and 

 the Calycophoridce and Physophoridte, of which the former constituted the 

 order Ciliograda, whilst the two latter made up the order Physograda. 

 The Ctenophora, however, are now generally placed amongst the Actinozoa, 

 whilst the Calycophoridce and Physophoridcz constitute the Hydrozoal sub- 

 class Siphonophora . 



