2 THE VOYAGE OF H.M.S. CHALLENGES. 



distinct heads from those without distinct heads, and, badly applying this character, 

 placed some without shells and some with shells in the same subdivision. Oken 1 

 did the same, placing Pneumonoderma and Clio in one class, and Clione and Cavolinia 

 in another class. 



It was not until 1824 that de Blainville, relying on very judicious anatomical con- 

 siderations, arrived at the same systematic result as Peron and Lesueur, and divided the 

 Pteropoda (which he named Aporobranchia) into Gymnosomata and Thecosomata. 2 He 

 rejected the strange forms that Peron and Lesueur had introduced into the Pteropod 

 group, but he left Phyttirhoe,* which formed in his classification a third group, 

 Psilosomata, of the same value as the two others. 



Other groupings based on characters drawn from one organ alone, led to very bad 

 results. Thus, Gray 4 divided the Pteropoda into Dactylobranchia and Pterobranchia, 

 the genus Cavolinia alone forming the first division and all the other genera being united 

 in the second. 



Rang, 5 who followed the Cuvierian subdivisions, through insufficient study of 

 characters placed some Pteropoda without shells and some with shells in the family 

 with a distinct head, and tried to do the same in the case of the group without a 

 distinct head. 6 This was, however, rather exaggerating the love of symmetry, although 

 not so much as Oken, 7 who desired that each order should contain four families, each 

 famdy four genera, &c. 



Latreille, 8 on the other hand, divided the Pteropoda according to the size of their 

 fins, into " Macropterygiens " and " Microptdrygiens." Pneumonoderma alone formed 

 the second group, while all the other forms were included in the first. 



Cuvier 9 and his school did not adopt the classification of de Blainville, but followed 

 the divisions established in the first edition of Le Regne animal. 



Since then, the non-natural arrangements have been gradually abandoned ; and the 

 division of the Pteropoda into two distinct groups, Thecosomata and Gymnosomata, is 

 now generally adopted. There is, however, a slight misunderstanding amongst some 

 zoologists as to the relative extent of these two subdivisions ; but we shall see further 

 on that this disagreement is really without foundation, and that the separation of the 

 two groups is very clear. 



When Cuvier established his division of Pteropoda, only two species (forming two 



1 Lehrbuoh der Zoologie, Bd. i., 1815. 



2 Diet, d. Sci. Nat., t. xxxii. p. 271. 



3 This animal remained here for a long time, until it was put in its right place, among the Nudibranchia. 



4 London Medical Repository, p. 235, 1821. 



6 Manuel de l'histoire naturelle des Mollusques, 1829. 



c Description d'un genre nouveau de la classe des Pteropodes, Ann. d. Sci. Nat., ser. 1, t. v. p. 284, 1825. 



7 Loc. cit. 



8 Les families naturelles du Regne animal, 1825. 



9 Le Regne animal, ed. 2. 



