INTERNAL STRUCTURE OF THE ASCIDIA. 43 



of these animals are so transparent as scarcely to be 

 seen while floating on the water ; and when captured 

 and examined^ the viscera may be readily distinguished 

 through their coverings. Their form is oval or cylin- 

 drical, and open at each extremity. Of these orifices, 

 one is large, and furnished with a valve, which permits 

 the ingress, but not the egress, of water, which, taken 

 into a cavity for the purpose of aerating the blood, 

 is expelled by the forcible contractions of the body 

 through the anterior aperture, so as to propel the 

 animal along, and in this manner it swims, proceeding 

 in a backward direction. The cavity in which the 

 water is received, is a wide membranous canal, travers- 

 ing the body from end to end ; and, as in the ascidice, it 

 is here that the blood is aerated, not, however, through the 

 medium of a diffused net-work of vessels, but by two 

 beautiful arborescent branchiae, traversing this tube on 

 each side. These ramified branchise are themselves 

 said by some naturalists to be tubular, and to open 

 into the common cavity, whence they receive the im- 

 bibed water ; but Cuvier denies this. 



The viscera occupy a cavity between the receptacle 

 for water and the soft gelatinous internal tunic, which 

 often glistens with the most brilliant hues. The mouth 

 is simple, and placed near the upper orifice, from which 



