NAKED-GILLED MOLLUSKS. 99 
ORDER NUDIBRANCHIATA. 
(Naked-gilled Mollusks.) 
SCARCELY any of the animals which the marine 
naturalist meets with in his dredging voyages, or 
his sea-side excursions, are so attractive as those 
of this Order. They are remarkably elegant in 
their forms, which present great variety. Their 
motions are graceful and lively, their colours 
peculiarly brilliant, and their history and economy 
marked by points of great interest. Nor is it 
rarely that the zoologist is gratified with an oppor- 
tunity of observing these very attractive Mollusks, 
for the species are numerous, the British shores 
alone yielding more than a hundred. 
These are not air-breathers, like the members of 
the preceding Order; they are all marine, and re- 
spire by means of gills, which are not enclosed 
within the body, but (at least during action) ex- 
posed freely to the surrounding medium. Some- 
times these organs are capable of being partially 
withdrawn into a cavity, situated on the medial 
line of the back ; but more commonly they take the 
form of simple or branched warts, arranged along the 
sides. The foot is large and broad. The mantle 
is ample, and projects above the foot on all sides. 
None of the species are protected by a shell, except 
in early infancy. ‘The young, on emerging from 
the egg, is enclosed in a shell, which closely re- 
sembles that of a nautilus, and bears very little 
resemblance to the parent in its form; it has, 
