540 CESIUM VENER1S ; SIPHONOPHORA. 



in the majority of the Order, but are wanting in the Beroe and 

 its allies, which resemble the Cydippe in general form. 



1193. The Cesium Veneris, or Girdle of Venus, belonging to 

 the same group with the preceding, is a flat riband-shaped 

 animal, which sometimes attains the length of five or six feet, 

 whilst its breadth is not more than as many inches. At first 

 sight the form of this species might forbid us from ranking it 

 near the Cydippe. When we examine its structure, however, 

 we find that there is a much greater real correspondence than 

 would have been suspected. The mouth is placed, not at one 

 extremity, but at the centre of one of the sides. The alimentary 

 tube passes straight across the body, and terminates on the 

 opposite side ; the digestive organs closely resembling those of 

 the Cydippe. The edges of both sides are fringed with cilia 

 from one end to the other ; and it appears to be by the vibra- 

 tions of these cilia, rather than by any movement of the body 

 itself, that it is propelled through the water. As in the Cydippe, 

 a system of vessels absorbs the nutritious fluid produced by the 

 action of the digestive apparatus, and conveys it to the remain- 

 der of the structure, here so remote. And here, also, we find a 

 part of these vessels running under the ridges bearing the cilia, 

 as if to expose the fluid they convey to a stratum of water con- 

 tinually renewed by the ciliary action. We might then regard 

 this animal as a Cydippe, flattened and extended in a lateral 

 direction ; and many very interesting forms of transition have 

 been discovered, which show that this view of its structure is 

 the true one. 



ORDER IV. SIPHONOPIIORA. 



1194. The Siphonophora are free-swimming compound ani-* 

 mals, which appear to be nearly allied to the Hydroid Polypes. 

 They present two very distinct types of structure, however, one 

 of which, that of the Physograde, or Hydrostatic Acalephce, is 

 characterised by the presence of one or more large air-sacs, by 

 which great buoyancy is given to these beautiful animals. It 



