378 PTEROrODA. 



have no claim to a higher distinction than that of forming' 

 a tribe of Gasteropods allied to Bulla and Aplasia, and 

 this opinion, which was advocated by De Blainville, has 

 recently been supported by Souleyet, a naturalist who has 

 had abundant opportunities for the study of them, and who 

 has done much to elucidate their history.* 



This is not the place to discuss the zoological status of 

 a group of animals playing so unimportant a part in the 

 British Fauna as the Pteropods do. Suffice it to say that 

 on account of their inferiority of organisation, and at the 

 same time the striking analogy they present with the 

 embryonic forms of the majority of marine gasteropoda, to 

 which in many of their features they have close relations 

 of affinity, we are induced to assign to them in this work 

 the position of an independent group superior to the 

 Acephala and below the Gasteropoda. 



The Pteropods are free and floating mollusca, swimming 

 by means of the wing-like fins already alluded to. Some 

 of them have shells and some are unprovided with such 

 coverings ; the latter have a more or less distinct head, 

 and a rudimentary foot, which are denied to the former. 

 The respiratory organs are external in the naked species, 

 enclosed in a cavity in those with shells. They have a 

 single heart. The viscera of their digestive system vary in 

 the different genera : some are provided with curiously 

 armed gizzards. The sexes are united. Their organs of 

 sense are very rudimentary ; the presence of otolitic ve- 

 sicles has been demonstrated by Souleyet. Their nervous 

 system is less highly developed than that of the Gaster- 

 opods. 



* See an abstract of M. Souleyet's Researches in the " Comptes Rendus " 

 for October, 1843. 



