CESTUM VENERIS. HYDROSTATIC ACALEPH^E. 477 



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, 6 and terminates on the 

 opposite side ; the digestive organs closely resembling those of the 

 Cydippe. The uniformly gelatinous consistence of the whole 

 animal, as well as this peculiar conformation of its alimentary 

 canal, forbid our ranking it in the Articulated series ; to which its 

 prolonged form might have led us to refer it, if the mouth had 

 been placed at one extremity, and the alimentary tube had 

 traversed its length. The edges of both sides are fringed with 

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

 vibrations 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 actions of the digestive apparatus, and conveys it to the 

 remainder 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 

 continually 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. 



1038. The group of Hydrostatic Acalephee, is characterised 

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

 buoyancy is given to these beautiful animals. It would appear 

 that they have considerable power over these organs ; either 

 forcing out the air contained in them, or compressing it into a 

 much smaller compass, when they wish to sink ; and distending 

 the sac by some unknown means when they desire to rise. 

 "We seldom meet with anything like radial symmetry in this 

 group. The forms of the species are extremely variable and 

 irregular ; but there is usually a correspondence between the two 



