CCELENTERATA : MEDUSID^. I2/ 



to employ the names Sertularida and Campanularida for the 

 sub-orders. 



ORDER V. THECOMEDUS^E. Professor Allman has recently 

 described under the name of Stephanoscyphus mirabilis, a very 

 remarkable Hydrozoon, which he believes to form the type of a 

 new order. This singular organism is invariably associated with 

 a species of Sponge, in the substance of which it is embedded. 

 It consists of a congeries of chitinous tubes, which permeate 

 the sponge-substance, and which open on its surface by large 

 openings resembling oscula. At their bases the tubes are 

 connected by horizontal branches, and they expand widely 

 as they approach the surface; where their contents become 

 developed into a remarkable body, which has the power of 

 extending itself beyond the mouth of the tube, and again of 

 withdrawing within it. This body is furnished with a crown 

 of tentacles, and is essentially medusiform in its structure. 

 There is a circular canal at the base of the tentacular crown, 

 surrounding the central opening, with four radiating canals 

 proceeding backwards from this ; but no lithocysts, ocelli, nor 

 velum have been detected. For this curious organism, Pro- 

 fessor Allman proposes the formation of a new order under the 

 name of Thecomeduscz. 



ORDER VI. MEDUSID^: or HYDROMEDUSID^E (Acalephce* in 

 part). The organisms included in this order have often been 

 separated as a distinct sub-class of the Hydrozoa ; but they 

 are, perhaps, best regarded as a mere order of the Hydroid 

 Zoophytes, characterised by the fact that the hydrosoma is free- 

 swimming and oceanic, consisting of a single swimming - bell 

 (" nectocalyx "\from the roof of which is suspended a single poly- 

 pite. A system of canals is developed in the walls of the swim- 

 ming-bell, and the reproductive organs are processes of the sides 

 of these canals or of the walls of the polypite. 



The Medusida comprise most of the smaller organisms com- 

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

 derived 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 old sub-class of the Acalephae contained the Gymnophthalmate 

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

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

 name of Pulmograda. The Acalephce also contained the Ctenophora and 

 the Calycophoridce and Physophoridce, 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 Physophorida constitute the Hydrozoal sub- 

 class Siphonophora. 



