A CRUSTACEA CHAP. 
the interest of the Crustacea lies. Before entering into an 
examination of these matters, it will be well to take a general 
survey of Crustacean organisation, to consider the plan on which 
these animals are built, and the probable relation of this plan 
to others met with in the animal kingdom. 
The Crustacea, to begin with, are a Class of the enormous 
Phylum Arthropoda, animals with metamerically segmented 
bodies and usually with externally jomted limbs. Their bodies 
are thus composed of a series of repeated segments, which are on 
the whole similar to one another, though particular segments 
may be differentiated in various respects for the performance of 
different functions. This segmentation is apparent externally, 
the surface of a Crustacean being divided typically into a 
number of hard chitinous rings, some of which may be fused 
rigidly together, as in the carapace of the crabs, or else 
articulated loosely. 
Each segment bears typically a pair of jointed mbs, and 
though they vary greatly in accordance with the special 
functions for which they are employed, and may even be absent 
from certain segments, they may yet be reduced to a common 
plan and were, no doubt, originally present on all the segments. 
Passing from the exterior to the interior of the body we find, 
generally speaking, that the chief system of organs which exhibits 
a similar repetition, or metameric segmentation, is the nervous 
system. This system is composed ideally of a nervous ganglion 
situated in each segment and giving off peripheral nerves, the 
several ganglia being connected together by a longitudinal cord. 
This ideal arrangement, though apparent during the embryonic 
development, becomes obscured to some extent in the adult 
owing to the concentration or fusion of ganglia in various parts 
of the body. The other internal organs do not show any clear 
signs of segmentation, either in the embryo or in the adult ; 
the alimentary canal and its various diverticula le in an 
unsegmented body-cavity, and are bathed in the blood which 
courses through a system of narrow canals and irregular spaces 
which surround all the organs of the body. A single pair, or 
at most two pairs of kidneys are present. 
The type of segmentation exhibited by the Crustacea is thus 
of a limited character, concerning merely the external skin with 
its appendages, and the nervous system, and not touching any 
