ARTICULATA. CRUSTACEA. 



SUB-CLASS I.-MALACOSTRACA* 



To this section belong almost all of the large 

 species, and many of the smaller, with which we are 

 most familiar. The appellation by which they are 

 distinguished can be considered only as comparative, 

 for their coverings are generally pretty solid, though 

 less hard than the true shells of the Mollusca. They 

 have either ten or fourteen feet, generally armed with 

 a nail. In the Order Decapoda, all the three pairs 

 of foot-jaws are appropriated to the mouth, and per- 

 form the office of true jaws ; but, in all the other 

 Orders, the hindmost and middle pairs of these 

 organs are used as feet, thus increasing the num- 

 ber to fourteen. In each case, two pairs of the 

 true jaws (maxillce) are covered by these changeable 

 appendages. The breathing .apparatus (branchiae) 

 are generally situated under the edges of the shell, 

 but in some they form tufts beneath the hinder part 

 of the body. The eyes, two in number, are for 

 the most part placed on a jointed and moveable foot- 

 stalk. 



There are five Orders in this section, of which we 

 can notice only the first and most important. 



* MAxo?, malakos, soft, and Sa-reetKov, ostrakon, a shell. 



