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-thoraz, 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, ¢, ¢’). All these are freely moveable upon one 
another, inasmuch as the exoskeleton which éonnects 
them is not calcified, but is, for the most part, soft and 
flexible, like the hard exoskeleton when the lime salts 
have been removed by acid. The mechanism of the joints 
will 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 
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