492 ARTHROPODA. 



fossil condition than is any of the other classes. All the four 

 great classes of Arthropods are, however, known to have been in 

 existence in rocks as old as the Silurian, while the Crustaceans 

 were thoroughly differentiated in the Cambrian period. It is, 

 therefore, clear that the point of divergence of the four primary 

 groups of Arthropods must be sought for in a period long anterior 

 to the Cambrian ; and it is hardly probable that we shall ever be- 

 come acquainted with the primitive form from which the " phylum " 

 of the Arthropoda took its origin. 



Class I. Crustacea. 



The class of the Crustacea may be generally denned as compris- 

 ing Arthropods which are essentially water-breathers, and usually are 

 provided with gills or branchial. The segments of the body are usu- 

 ally definite in ?iumber, and some of them almost invariably carry 

 jointed appendages. The head, typically, carries two pairs of anten- 

 na ; some of the appendages are modified to act as masticating or- 

 gans ; and the segments of the abdomen are commonly furnished with 

 locomotive appendages. The body is protected by a chitinous or 

 partially calcified exoskeleton or "crust," and the number of am- 

 bulatory limbs is mostly from five to seven. The Crustacea, in 

 general, pass i?i development through a distinct metamorphosis, but 

 the nature of this differs in different cases. 



The body of a typical Crustacean, such as a Lobster, generally 

 exhibits a division into three regions — a head, a thorax, and an 

 abdomen — each of which is composed of a certain number of 

 somites, which may be free, or may be more or less indistinguish- 

 ably fused with one another. Very commonly, the segments of 

 the head and thorax are united together into a " cephalothorax," 

 which may be protected by a common shield or " carapace " 

 (fig. 352, ca). The segments of the abdomen may be separate 

 and movable on one another, or a smaller or larger number of 

 the terminal segments may be fused to form a caudal shield or 

 "pygidium." The last segment of the abdomen (fig. 352, t) 

 is known as the "telson," and is generally without appendages, 

 while the anus opens on its lower surface. The " telson " has been 

 variously regarded as an unpaired appendage, as a segment devoid 

 of appendages, or as representing an aborted region of the body, 

 the latter being the view which, on various grounds, seems to be 

 the most probable one. 



Each segment of the body of a Crustacean may be regarded 

 as essentially composed of a convex upper plate ("tergum"), 

 closed below by a flatter ventral plate (" sternum "), the line 

 where the two unite being produced downwards and outwards, or 



