$30 ON THE CLASSES OF THE 



united in the same manner as originally they were in the 

 ovary, and swimming together in long chains, where the in- 

 dividuals are always disposed in a particular manner, ac- 

 cording to the nature of the species. 



The simple Tunicata possess a distinct branchial system 

 of respiration, with a liver, heart, and in fine a complete 

 system of circulation, which corresponds with that of the 

 Mollusca. In the system of generation also, of both these 

 groups, no small affinity may be discerned, at least the 

 ovary of Salpa scutigera bears a remarkable resemblance 

 to that of some Gasteropoda- But in order to obtain a 

 distinct view of the progress we have made, it may be 

 proper to give here a rapid sketch of the discoveries of 

 Cuvier, detailed in his inimitable anatomical Paper on the 

 structure of the Ascidia, or Thethya of Aristotle. 



The body of one of these animals is divided into three 

 cavities. First, the branchial, which communicates directly 

 with the air at its upper aperture, and at the bottom o* 

 which is the true mouth or entrance to the intestinal canal. 

 Secondly, the peritoneal cavity, which does not communi- 

 cate directly with the open air, but which is traversed by 

 the intestinal canal, originating in the branchial cavity and 

 passing along by means of the rectum towards the anal 

 aperture. Thirdly, the pericardial cavity, inclosing the 

 heart, and communicating neither directly nor indirectly 

 with the atmosphere. 



The Tentacula of the Polypes are still visible, but per- 

 form a new office in the Ascidite. They are here no longer 

 instrumental in catching the prey, but appear reduced to 

 be simple auxiliaries of the system of respiration; they 

 surround in fact the branchial cavity, and not the mouth 

 properly so called, which is a small opening without lips 



