254 



CRUSTACEANS. 



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 exopodite, as well as the principal branch or 

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



schizopod stage. Lastly, the median eye and the 

 exopodites of the motor-thoracic limbs disappear, and 

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

 ever, exceptional amongst 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 Zosea form, although characterised 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 moults 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. 



ZO.EA. stage of crab (enlarged). 



The Typical Crustaceans, — Subclass 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 body is very generally nineteen (but never more), and each has a pair of 

 appendages. 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 



JAWS OF CRAYFISH. 



a, Mandible ; b, c, Maxillae ; d, e, f, 1st. 2nd, and 3rd Maxillipedes. 



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

 the shield that covers it the carapace. Sometimes too, as in the crayfish, the 

 anterior three pairs of thoracic appendages are transformed into jaws, and on this 



