72 PTEROPODA. 
very fragile; nearly in the form of a triangular 
pyramid; with an aperture at its base, from which 
proceeds a long and slender glassy spine; and a 
similar spine projects from each side of the middle 
of the shell. The hinder part of the animal is 
globular and pellucid, and in the dark vividly 
luminous, presenting a singularly striking appear- 
ance, as it shines through its perfectly transparent 
lantern. Both of these are found floating in great 
numbers on the surface of the tropical sea. 
Others are entirely destitute of a shelly covering, 
as is that little species which occurs in enormous 
profusion in the Arctic Seas, and which we now 
proceed to describe. 
GENUS CLIO. 
(“ Whale-food.”) 
These little creatures have an oblong mem- 
branous body, without a mantle; a head formed of 
two rounded lobes, each of which is furnished with 
three long tentacles, capable of being withdrawn 
into a fold of skin, or protruded at pleasure. The 
mouth, which is terminal, has two small fleshy lips ; 
and two eyes, of elaborate structure, are placed at 
the back of the neck. 
The species best known is that which is com- 
monly called by our northern voyagers, Whale-food 
(Clio borealis). Though not more than an inch in 
length, it occurs in such countless millions as to 
form the principal part of the nourishment required 
by the most gigantic of living creatures. The Clio. 
bears some slight resemblance to a butterfly just 
emerged from the chrysalis, before the wings are 
