CCELENTERATA : HYDROZOA. 95 



phyllia, the latter arranged with the other organs in a continuous series. 

 Ccenosarc filiform. Pneumatocyst small. 



Fain. III. Physophoriadce. Hydrosoma with nectocalyces, but without 

 hydrophyllia. Distal end of the filiform coenosarc dilated. Pneumatocyst 

 small. 



Fam. IV. Athorybidce. Hydrosoma without nectocalyces, but with 

 hydrophyllia. Pneumatocyst occupying almost the whole of the globular 

 coenosarc. 



Fam. V. Rhizophysiada. Hydrosoma without either nectocalyces or 

 hydrophyllia. Coenosarc filiform. Pneumatocyst small. 



Fam. VI. Physaliadte. Pneumatocyst occupying almost the whole of 

 the thick and irregularly fusiform coenosarc. No nectocalyces or hydro- 

 phyllia. 



Fam. VII. Velellidtz. Hydrosoma without nectocalyces or hydro- 

 phyllia ; 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 

 discoiclal crenosarc. 



CHAPTER X. 



DISCOPHORA. 



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

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

 single definition necessarily suffices for both. The Medusidce. 

 are denned 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 im- 

 probable - that it will ultimately be entirely done away with, 



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

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

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

 name of Pulmograda. The Acalefihtz also contained the Ctenophora and 

 the Calycophonda and Physophoridcz, 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 Acfinozoa, 

 whilst the Calycophoridce and Physophorida constitute the Hydrozoal sub- 

 class Siphonophora. 



