405 
of Gasteropodous Mollusca. 
an order, so remarkable for the beauty and variety of the branchial 
appendages with which the species are adorned. The mollusks 
now under consideration are, on the contrary, distinguished by 
the extreme simplicity of external form, and by the absence of 
any specialized breathing organs. It would, therefore, be more 
in conformity with the views on which the existing orders of 
Gasteropoda were established by Cuvier, to consider this group 
as forming a separate order, characterized by the absence of spe- 
cialized branchiae ; and as the function of respiration is entirely 
performed by the skin, we propose to call this order Pelli- 
BRANCHiATA, and to include in it the following genera : — 
Elysia^ Pisso [Actaon^ Oken). 
Limapontia, Johnston {ChalidiSj Quatrefages) . 
Acteonia, Quatrefages. 
Cenia, Alder and Hancock. 
The Placobranchus of Van Hasselt, a genus involved in great 
obscurity, may possibly belong to this order, as it has a very 
evident relationship with Elysia ; but, as it is stated to have la- 
mellated branchiae, disposed on the back and lateral lobes, we 
think it more probable that it is an aberrant group of the Nudi- 
branchiata, forming a passage to the present order through the 
genus Elysia, the latter being itself a slight departure from the 
more simple form of the Pellibranchiata. 
On the other hand, this order is nearly allied to the Infero- 
branchiata through a small mollusk which we have already de- 
scribed in the ^ Annals of Natural History*,^ having very much 
the form and appearance of the Pellibranchiata, but possessing 
external plumose branchiae, under the right side of the cloak, as 
in the former order, to which it must consequently be referred. 
This animal we take to belong to the genus Pelta of Quatrefages, 
though the characters he assigns to that genus differ in many 
respects from those we find in our species. 
In consequence of the extreme degradation from the molluscan 
type that M. de Quatrefages has seated to exist in these little ani- 
mals, they have become objects of much interest in a physiolo- 
gical point of view, and we therefore consider ourselves fortunate 
in having the opportunity of examining two or three of the ge- 
nera, and especially in having got one of these in such great 
abundance, as, notwithstanding its minuteness, to enable us to 
make out its anatomy very satisfactorily. The anatomy of this spe- 
cies {Limapontia nigra) we now purpose giving in detail, and from 
the slight examination we have been able to make of the other two 
genera, species of which we have described, we believe it may be 
taken as a fair example of the anatomical characters of the order ; 
* Vol. xviii. p. 289. 
