4 CRUSTACEA 



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 tbe enormous 

 Phylum Arthropoda, animals with metamerically segmented 

 bodies and usually with externally jointed 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 limbs, 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 lie 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 



