THE TYPICAL CRUSTACEANS 



3221 



bers of the latter group, nevertheless, it does occur, as in one of the shrimps 

 (Pen<zus~). In this the Nauplius passes into a stage called the Zocea, in which four 

 pairs of appendages, representing the maxillae and the two following pairs of limbs 

 of the adult, have appeared, and the abdominal region has increased in length, 

 although, like the greater part of the thorax, is still limbless. A pair of compound 

 eyes is present on the sides of the head. After this so-called copepod stage, the 

 large eyes become stalked, the abdomen continues to increase in length, and takes 

 on the function of swimming, which was before performed by the antennae, and 

 the remainder of the thoracic and abdominal limbs appear. Since the thoracic limbs 

 are provided with a distinct exopdite, as well as the principal branch or endopo- 

 dite, as in the cleft-footed shrimps (Schizopoda), this larva is known as the 

 schizopod stage. L,astly, the median eye and the 

 exopodites of the motor-thoracic limbs disappear, and 

 the adult form of the Penczus is attained. It is, how- 

 ever, exceptional among the higher forms for the 

 young to be set free in the Nauplius stage. The young 

 of the lobster, for instance, hatches in the schizopod 

 condition; while that of the common crab appears in 

 the Zoaea form, although characterized by the presence 

 of a long dorsal spine, and a sharp beak on the carapace. 

 Moreover, the two pairs of antennae, the mandibles, and 

 maxillae, are of small size, while the following two pairs 

 of limbs are relatively large, and forked. By means 

 of these the minute transparent creature swims, and 

 after undergoing several molts passes into a stage termed the Megalopa, which is 

 much like the adult, but has enormously large eyes, and swims by flapping its long 

 jointed abdomen like a shrimp. 



STAGE OP CRAB. 



(Enlarged.) 



THE TYPICAL CRUSTACEANS SUBCI^ASS Malacostraca 



Much difference of opinion still obtains as regards the classification of Crus- 

 taceans, which are here divided into two main subclasses. In the present group, 

 comprising the largest and most familiar forms, the number of segments in the 



JAWS OP CRAWFISH. 

 a. Mandible ; b, c. Maxillae ; d, e,f, ist, 2nd, and 3rd Maxillipedes. 



body is very generally nineteen (but never more), and each has a pair of append- 

 ages. The first five segments compose the head, which, except in some blind 

 species, bears a pair of compound eyes, two pairs of antennae, and three pairs of 

 jaws, namely, a pair of mandibles in front, and two pairs of maxillae behind. The 

 eight segments behind the head, which constitute the thorax, may be united with 

 the head, as in crabs, when the whole region is termed the cephalothorax , and the 



