08 CRUSTACEA. 



The consolidation of a few of the anterior rings has pro- 

 duced in many genera a cephalic or mouth region. The 

 skin of the segmented body is covered with a chitinous 

 cuticle, and bears either seta;, or unjointed appendages, 

 or both. Respiration is effected by the general surface 

 of the body, and by the appendages, and no heart com- 

 parable with that of the Crustacea is present. Adult 

 size is attained by the development of rings between 

 those already formed near the posterior end of the 

 body, i.e. between terminal and subterminal rings. 



In the Crustacean, on the other hand, the number of 

 segments is less variable and the full complement is at- 

 tained in the early stages of growth. The body is short- 

 ened, and the two ends are not only distinct in function, 

 but very different in form. A coalescence of a number 

 of the anterior rings has produced a complicated ce- 

 phalic or mouth region, and in the higher forms a ceph- 

 alothoracic region. The segmented body in most forms 

 is covered with a hard crust or shell, and bears jointed 

 appendages. Respiration is usually carried on by gills, 

 a heart is generally present, and growth is made possi- 

 ble by repeated moults of the unyielding shell. 



Figs. 31 and 32 are views of the horse-shoe crab 

 [Limulus], viewed from the upper and under sides, 

 showing the three regions of the body and the mouth 

 between the bases of the walking legs. This animal is 

 not properly a Crustacean, but the figures of it have 

 been given here because it is referred to in many text- 

 books as a Crustacean. The structure will be noticed 

 in the Guide which treats of Millepods, Scorpions, and 

 Spiders. 



Typography by J- s. Cushing & Co., Rostov, i.'.s.a. 



