978 Marvels of the Universe 
CRAB FORM AND 
HABIT 
BY EDWARD STEP, F.L.S. 
ONE of the most engross- 
ing studies in natural 
history is the considera- 
tion of the way in which 
Nature, without altering 
the general structure, 
adapts the various mem- 
bers of a group to different 
habits and habitats. It 
does not matter whether 
you take beasts, birds, 
fishes, or any other of 
the principal classes of 
animal life, you will find 
similar methods adopted 
to produce the vast variety 
of forms with which we 
are acquainted. All the 
backboned creatures are 
built on essentially the 
Photo bu) LE. Step. E , 
LONG-LEGGED SPIDER CRAB. same plan; yet by the 
The limbs are drawn out to great length and fineness, so that the Crab can walk extra development or the 
and: climb with ease over the most delicate seaweeds. partial suppression of cer- 
tain bones, or sets of bones, the utmost variety of external form is brought about, and, with it, 
adaptation to a special mode of life. 
The same rule holds good with other classes. Here is the great Crab family—the Crustacea, 
so-called because their outer skin is so impregnated with lime, that it forms a hard, unyielding box, 
which does away with the necessity for a complex internal skeleton. Owing to the shell-like 
character of the integument, the Crabs have been lumped with oysters and whelks, etc., under the 
name of Shell-fish—one of the most unfortunate and misleading terms ever invented, because none 
of the diverse forms included under it is a fish. In spite of this hard, unyielding box, which has 
to be thrown off entirely from time to time to allow of the creature’s increase of size, the type is seen 
to be perfectly plastic. Because it is the best-known crab-form, let us take the common Jack Crab 
of the fishmongers’ shops as the type, and point out a few of the directions in which it has been 
modified to adapt it to habits differing, more or less, from those of the Jack Crab. This familiar 
crab has the principal part—the trunk—broad in front and narrowed behind, and to its under- 
side are attached five pairs of jointed legs. There is also a thin hinder part, popularly known as 
the “tail,” which attains a considerable size in’ the Lobsters, but in the Crabs is usually folded 
underneath to be out of the way. The foremost pair of legs are so hugely developed, that they are 
of no use for locomotive purposes, but of great value for defence and offence, the last segment ending 
in a powerful pair of nippers, with tooth-like knobs and points on their opposed edges. The other 
four pairs of legs are used for running over the sea-bottom, and their last joints are almost solid, 
sharp points, useful for digging into sandy or gravelly floors and retaining a firm hold. The wedge- 
like form of the body, viewed from the side, is useful for thrusting backwards under a rock, where, 
