44*0 PHILOSOPHY OF ZOOLOGY. 



grey ; the suckers whitish." The specimens which furnish, 

 ed an opportunity for making the preceding observations, 

 were met with in the Gulf of Guinea, and afterwards on 

 the voyage, swimming in a small argonauta, on the surface 

 of the sea. The reader, who is desirous of farther infor- 

 mation on this subject, may consult Dr Leach's Observa- 

 tions on the Genus Ocytlioe ofRafinesque, and Sir E. Home 

 on the DistinguisMng Characters between the Ova of the 

 Sepia, and those of the Vermes Testacea that live in water, 

 in the Philosophical Transactions for 1817, art. xxii. and 

 xxiii., (both of which are added to the appendix of Captain 

 Tuckey's Narrative), and a paper by Mr Say, on the 

 Genus Ocythoe, in the Phil. Trans. 1819, art. vii. 



Class II. — Pteropeda. 



Fins formed of membranaceous expansions. 



This class was instituted by Cuvier, for the recep- 

 tion of a few genera, the peculiar characters of which 

 indicated the impropriety of suffering them to remain in 

 any of those categories which had been previously esta- 

 blished. All the species are small in size ; and the attempts 

 hitherto made to investigate their internal structure, have, 

 in a great measure, failed in explaining the functions of the 

 organs which are exhibited. The valuable papers of Cu- 

 vier, on the Clio, Pneumodermon and Hyalea, include 

 nearly all the accurate information on the subject, of which 

 naturalists are in possession. 



The general form of these animals is somewhat ovate. 

 The tunic appears in some genera, as the Clio and Pneu- 

 modermon, to be double, the external one soft and thin, the 

 internal exhibiting a fibrous structure, corresponding to the 

 muscular web of the skin of the hio-her classes. In these 

 annnals, however, these two layers are unconnected through- 

 out the greater part of their expansion. In some, as the 

 Cymbullia, the tunic is cartilaginous, while in others it is 



