132 BY THE DEEP SEA. 



The Crustacea belong to that grand division of the animal 

 kingdom known as the Arthropoda, i.e., animals whose bodies 

 consist of a series of variously-shaped segments, the skeleton 

 being external, and giving more definite form to those rings, 

 which are placed edge to edge, and some of which have limbs 

 attached to them. Taking a bird's eye view of the crabs, and 

 seeing only the continuously solid surface of the carapace, it 

 would be difficult to accept this statement; more especially 

 should we stare hard at the crab's back if we were told that 

 the typical number of such rings or segments in the Crustacea 

 is twenty (some authorities say twenty-one). But if we turn 

 the crab over so that we can get a fair view of his smooth white 

 underside, we begin to think there may be something in this ring 

 theory after all, for the undercrust is not solidly continuous 

 like the upper, but marked off by grooves to indicate the seg- 

 ments. The idea is that in the original progenitor of the race 

 the whole twenty segments were distinct and had independent 

 movement, but that in the process of evolution of the various 

 species it has served their purpose in life to have some of these 

 segments soldered together. And so in the many genera into 

 which the vast army of crustaceans are classified, we find 

 great variations in this respect ; also in the various functions 

 which the pair of limbs or otherwise modified appendages that 

 spring from each segment is called upon to play. 



Under the carapace of the Great Crab are gathered together 

 no less than fourteen segments, nine belonging to the head 

 arid bearing appendages transformed into eyes, antennae, jaws, 

 etc. ; whilst five belong to the trunk and bear the great chela: 

 and the four pairs of walking limbs. The remaining six seg- 

 ments belong to the tail (pleon), and in the crabs are folded over 

 under the united head and trunk. Among the different groups 

 of Crustacea we shall find the widest variations in the arrange- 

 ments of these parts; even in different genera of crabs is 

 this so, as we shall see before we bave left the long drang. 

 " Glancing along the whole line of limbs, as the outgrowths 



