142 AQUATIC ORDERS. 



above. These animals, by some means not understood, 

 are capable of distending the body, by filling it with 

 water. 



The Carinaria, of which one species (Carinaria cym- 

 hium) is found in the Mediterranean sea, may be se- 

 lected as an example. 



Cuvier's sixth order is termed Pectinihranchiata, (hav- 

 ing fringed or pectinated branchiae,) and is without com- 

 parison the most extensive of all : it includes all the 

 inhabitants of spiral shells, inhabiting the sea. Like 

 the Pidmobranchiata, the respiratory organs are on the 

 roof of an internal cavity, occupying that part of the 

 body within the last or widest whorl of the shell : this 

 cavity communicates with the water, by means either of 

 an orifice, or a syphon formed by a fold of the mantle ; 

 the branchiae usually constitute a double or triple line 

 of fringe-like appendages. In some, however, there is 

 only a single row ; and in two genera, the roof of this 

 cavity, instead of having these fringes, has its surface 

 lined with an exquisite vascular net-work. But these 

 genera, namely, Cydostoma and Helecina^ respire air; 

 the first is terrestrial in habits, the second aquatic. 

 M. Ferussac ranges these genera with the pulmobran- 

 chiata, and perhaps correctly. 



From the Pectinihranchiata a group has been sepa- 



