70 



ACALEPtLE. 



1. 



Fig. 25. 



dently deposited in 

 thin secondary lami- 

 nae, which gradually 

 increase in size as the 

 animal advances in 

 growth, the inferior 

 being the largest and 

 last formed. When 



examined after its removal from the body, this fragile skeleton is 

 seen to be extremely porous or cellular ; and, the pores being 

 filled with air, it is specifically lighter than water, a circumstance 

 which may contribute to the buoyancy of the animal, even when 

 alive. 



The lower surface of Porpita is furnished with numerous appen- 

 dages called cirri, some of which appear to be organs of prehension, 

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

 principal agents in progression ; yet in other Cirrigrada, as Velella 

 and Rataria, besides the horizontal lamella, which 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, which 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, apparently of a muscular nature, 

 by the contractions of which the sail can be lowered or elevated at 

 pleasure. 



(98.) Diphyda. The last family of acalephse derives its name 

 from the singular appearance of the creatures which compose it : 

 each animal, in fact, seems to consist of two portions so slightly 

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

 nature of the con- 

 nection which ex- 

 ists bet ween them; 

 and from the per- 

 fect transparency 

 of their bodies, 

 which is such that 

 it is with great 

 difficulty they are 

 discoverable even 

 in small quanti- > 



