CIKRIOKADA. DIPHYEA. 127 



but perform also the office of oars, which, in this species, are the prin- 

 cipal agents in progression; yet in other Cirrigrada, as Velella and 

 Rataria, besides the horizontal lamella that forms the whole skeleton 

 of Porpita, there is a second subcartilaginous plate, rising at right angles 

 from its upper surface, and supporting a delicate membranous expansion, 

 that rises above the water and exposes a considerable surface to the 

 wind, so as to form a very excellent sail. To perfect so beautiful a 

 contrivance, in Rataria the crest is found to contain fibrous bands, ap- 

 parently of a muscular nature, by the contractions of which the sail 

 can be depressed or elevated at pleasure. 



(329.) DIPHTEA. The last family of the Acalephae derives its name 

 from the singular appearance of the creatures composing it ; each ani- 

 mal, in fact, seems to consist of two portions (a, 6, fig. 63, 1, 2) so 



Fig. 63. 



Diphyes Bory. 



slightly joined together, that it is by no means easy to understand the 

 nature of the connexion between them. 



(330.) The body of these strangely-organized beings is composed of 

 two polygonal, subcartilaginous, transparent pieces placed one behind 

 the other, the posterior division being implanted more or less deeply into 

 the anterior. These two divisions are invariably more or less dissimilar 

 from each other ; nevertheless they oifer this circumstance in common, 

 that they are excavated internally by a deep cavity, which opens exter- 

 nally with a wide orifice of regular shape, although differing in form in 

 each division. To these details of their general appearance must be 

 added the existence of a long filiform appendage, which issues from 

 the upper cavity of the anterior cartilaginous portion, and which was 

 regarded by Cuvier as the ovary. 



(331.) On more minute examination, there is recognizable in the 

 anterior division a visceral mass called the nucleus, which is made up of 

 a proboscidiform oesophagus, terminated by a sucker-like mouth, and 

 continuous with a stomachal cavity, whereunto are appended hepatic fol- 

 licles of a greenish colour, and sometimes a little vesicle filled with air. 



(332.) Besides the above structures there may be remarked, towards 

 the lower part of the body, another glandular-looking mass, probably 



