452 TRANSFORMATIONS OF INSECTS. 



parts of the crab, from their simplest to their adult and most 

 elaborate condition. 



The embryo, when it first quits the &gg, is enclosed within a 

 general tunic, which embraces each individual member, and is 

 conformable to the whole animal. The tunic is exceedingly trans- 

 parent, and the hairs of the young immature crab and the horn- 

 like processes which characterise the carapace, are folded down 

 and compressed. The crab swims about for a few hours in this 

 state, and then liberates itself by throwing off the portion of tunic 

 which covers the head and the part of the body which gives off 

 the long legs. The tunic upon the jointed tail-like end is cast off 

 some hours afterwards. (Page 454, Fig. i.) 



The following terms are used to distinguish the portions of the 

 body of the Crustacea. The tail-like long body is called the pleon, 

 from irAeo), to navigate, being that part of the animal which sup- 

 ports the limbs by which it swims about ; and the swimming 

 limbs are called plcopoda. The tail end is the telson. The portion 

 which supports the walking legs is called the pereion, from Trepatoco, 

 to walk about ; and the legs are termed percipoda. This portion 

 is hidden by the carapace in the young state, and this last-named 

 part is what is familiarly considered to be the head. 



When freed from the tunic, the animal sports and gambols in 

 the water partly by the action of the entire pleon, and partly by 

 the great feeler legs attached to the pereion. The carapace is seen 

 to be armed with a long projecting beak or rostrum, and a spine- 

 like portion projects from the back. (Fig. on page 453.) In this 

 early stage the dorsal spine or that on the back, is no mere 

 ornament, but possesses a specific use in supporting the heart in 

 its position. The carapace is attached to the pleon by a mem- 

 brane, and there is an opening in it on either side, so as to 

 admit a stream of water into the branchial cavities. The cara- 

 pace is compressed in front, and the eyes are prominent, and are 

 fixed upon a short stalk. 



In the youngest stage, and in the crab when escaped from the 

 tunic (Zo'ca — for the crabs, unlike many other Crustacea, do not go 

 through the naupliiis form) there is a pair of appendages in front 

 which represent the antennae, and there are some hairs upon them, 

 which become gradually developed into the organ of hearing. 



