MOLLUSCA —CLASS IV. PTEROPODA. ihe 
together, and it has been observed that they rarely make their appearance 
in the daytime, or in stormy weather. 
In regarding their distinctive features of organization, the Pteropoda 
are clearly intermediate between the Gasteropoda and the Cephalopoda ; 
the foot or gastropodal disc of the former, being modified into a kind of 
pteropodal natatory fin, offers an intermedial transition as it were to 
the locomotive tentacula of the latter. They are of soft and somewhat 
gelatinous structure, and have the abdominal portion of the body in- 
closed within a glassy shell; there is a rather indistinct head, almost or 
altogether destitute of eyes, and a mantle which is large, thin, and ca- 
pable of great dilatation and contraction. The mouth is subterminal, 
and it is provided on each side with one or more membranaceous wing- 
like natatory fins. The breathing organs are pectinated, and internal, 
like those of the Gastropods ; but in some species they are so exceed- 
ingly minute, as to be only just discernible under a strong magnifying 
power. 
The conchiferous Pteropods (for they are not all conchiferous) have 
sometimes a globose, sometimes a cylindrical shell, and it is for the most 
part either partially or altogether enveloped by the mantle ; in some cases 
the shell, however, becomes modified into a kind of gelatinous cartilagi- 
nous integument. 
The rank as well as the situation which we have assigned to these 
mollusks in the natural system, is that latterly adopted by Cuvier; Gray 
pursues the same method of arrangement, but not, however, Lamarck or 
De Blainville*. ‘They were placed by the learned author of the ‘ Histoire 
des Animaux sans Vertébres’ between the Brachiopods and the Gastro- 
pods, under an impression that the wing-like pair of natatory fins is 
merely a modification of the bilobed mantle of his ‘ Conchiféres,’ and 
the shell of Hyalea a modification of the shell of Terebratula. Here, 
* De Blainville’s crude notion of the Argonaut appears to have somewhat influenced his 
methodical distribution of the Pteropods. The genera Argonauta and Spiratella are asso- 
ciated together in his ‘Manuel de Malacologie’ under the title of ‘Les Pteropodes,” 
the rest of this class are placed after the Bulle, Aplysia, &c., in another and separate family, 
whilst 
under that of “ Les Aporobranches.” 
