THE EXOSKELETON. 19 



is a firm and solid front part, covered by a large con- 

 tinuous shield, which is called the carapace ; and a jointed 

 hind part, commonly termed the tail (fig. 2). From 

 the perception of a partially real, and partially fanciful, 

 analogy with the regions into which the body is divided 

 in the higher animals, the fore part is termed the cepha- 

 lo-thorax, or head (cephalon) and chest {thorax) com- 

 bined, while the hinder part receives the name of 

 abdomen. 



Now the exoskeleton is not of the same constitution 

 throughout these regions. The abdomen, for example, 

 is composed of six complete hard rings (fig. 2, xv-xx), 

 and a terminal flap, on the under side of which the 

 vent (fig. 3, a) is situated, and which is called the telson 

 (fig. 2, t, i'). All these are freely moveable upon one 

 another, inasmuch as the exoskeleton which connects 

 them is not calcified, but is, for the most part, soft and 

 flexible, like the hard exoskeleton when the Ume salts 

 have been removed by acid. The mechanism of the joints 

 wUl have to be attentively considered by-and-by ; it is 

 sufficient, at present, to remark that, wherever a joint 

 exists, it is produced in the same fashion, by the exo- 

 skeleton remaining soft in certain regions of the jointed 

 part. 



The carapace is not jointed ; but a transverse groove is 

 observed about the middle of it, the ends of which run 

 down on the sides and then turn forwards (figs. 1 and 2, 

 eg). This is called the cervical groove, and it marks off 



