442 PHILOSOPHY OF ZOOLOGY. 



The intestine is short, and, after making one or two turns, 

 ascends and terminates in the neck near the mouth. 



The circulating system in this class has been but very 

 imperfectly investigated. The pulmonic vessels are un- 

 known, but systemic veins, a single auricle, ventricle, and 

 aorta, have been detected. The heart, in some, is situate 

 on the left, in others, on the right, side of the body. 



The aerating organs exhibit very remarkable differ- 

 ences. In the Clio they are in the form of a fine net- 

 work on the surface of the fins; in the Pneumodermon 

 they are conjectured to form leaf-like ridges on the caudal 

 extremity of the body ; or if these ridges are to be consi- 

 dered as particular kinds of fins, the gills may be sought 

 for on the membranaceous expansions of the neck. In the 

 Hyalea the branchiae form a complex band on each side of 

 the body, at the lateral opening of the shell. 



The animals of this class are all hermaphrodites. There 

 is 'a common cavity, a vesicle, penis, vas deferens and testi- 

 cle, together with an oviduct and ovarium. These open 

 near the mouth on its ventral margin. There is nothing 

 known with respect to the appearance of the eggs, the pe- 

 riod of propagating, or the form of their young. 



All the animals of this class inhabit the sea. Some of 

 those, as the Clio and Limacina, frequent the arctic regions, 

 and afford the whale a great part of its sustenance. None 

 of the species of the class have hitherto been detected in 

 the British seas. 



M. CUVIER divides the animals of this class into cepha- 

 lous and acephalous. In the latter division he places the 

 genus hyalea. The head of the animal of this genus, with 

 its inferior neck, may, however, be sufficiently recognised 

 to remove all doubt of its existence. 



The characters which may be employed in the classifica- 

 tion of this group are numerous ; but the influence which 



