THE LIVING SPECIMENS IN THE AQUARIUM. 105 
generally of a purplish red, sometimes of a bright orange color, and is of 
the form called multivalve, being composed of five pieces or valves, two 
of them on each side of the animal, and a narrow piece down the back. 
It is a pretty shell, clear and brittle, of a white color, tinged with pale 
blue. 
Tne Common Crab is too well known to require a description of its 
peculiarities of form; but there are interesting facts connected with the 
history of its species which are not 
commonly understood. Some few 
species of crabs penetrate to a con¬ 
siderable distance inland, but are 
compelled to return to the sea at 
the period of spawning. Among 
the marine species, the majority do 
not quit the shores, whilst others 
are found at great distances in tho 
high seas, where they can rest only 
on the floating banks of sea-weeds, 
so abundant in the tropics. Some 
species again frequent only the 
rocky parts of the coast, while others prefer sandy shoals, in which they 
bury themselves. 
The most remarkable facts in connection with their history, are their pe¬ 
riodical moulting, and the recently discovered metamorphoses which they 
undergo in the early stages of their existence. Some very curious creatures, 
long regarded by naturalists as a distinct genus, zoea, have been discovered 
to be the larvae of crabs, into which they ultimately become transformed. 
These transformations take place when the crabsuere of very diminutive 
size, little larger than a flea; and 
hence they are not open to ordinary 
observation, but may be seen with 
the aid of a microscope, or a powerful 
magnifying-glass. At first the young 
crab appears with a curiously hel- 
meted head, surrounded by numerous 
lengthy processes, and having large 
sessile eyes, Fig. 1; in the second stage 
it presents more of tire crab-like char¬ 
acter, the abdominal organs being 
more fully developed, Fig. 2. A 
similar metamorphosis takes place 
amongst both the highest classes of 
Crustacea; and in their earlier stages of development they very nearly 
resemble each other, though they may differ very widely when fully 
developed. 
COMMON CBAB. 
5* 
