THE CLASS CRUSTACEA 319 



in one or both sexes of which some of the abdominal 

 appendages are suppressed. 



The appendages of the head are the eyes, which are often 

 carried on stalks ; the antennules, which are branched and 

 contain in their basal segment an otocyst or statocyst ; the 

 antennae, on the base of which the excretory organs open ; 

 the mandibles, which are strong and tooth-like; the 

 maxillules, and the maxilla. The last two are branched, 

 lobed, setose plates and lie like leaves on the mandibles. 



The appendages of the thorax are not so diversified as 

 those of the head. The first three pairs are more or less inter- 

 mediate in structure between jaws and legs : they are crowded 

 together in front of the mouth and are known as foot-jaws, or 

 maxillipeds. The last five pairs are strong crawling-legs ; any 

 of them may end in nippers (chela;), and one of them — the 

 first or second — may be enormously enlarged and may end 

 in particularly large and powerful chelae for tearing and 

 grasping food. 



The appendages of the abdomen are, usually, biramous, 

 feathery, or leaf-like swimming-organs. Those of the 6th 

 segment are, commonly, of pre-eminent size, and form with 

 the terminal limbless segment, or telson, a large tail-fan, 

 with which, by powerful strokes of the abdomen, the animal 

 swims backwards. In most Decapods the first two pairs of 

 abdominal appendages of the male are modified in some way 

 for sexual purposes, and many of the abdominal appendages 

 of the female are used for carrying the eggs, being sometimes 

 used exclusively for this purpose {e.g. in crabs). 



The capacious stomach of the Decapods is furnished with 

 strong calcareous grinders and is a masticatory organ. 



The reproductive organs open either on the basal segment 

 of a pair of the thoracic legs, or on the sternum in that vicinity 

 — those of the male on the last pair, those of the female on 

 the antepenultimate pair. 



The Decapoda may conveniently be marshalled in three 

 series— Macrura, Anomura, and Brachyura. 



The Macrura (/>ia/c/oo's = long, and oi5pd = tail) are the 

 Prawns, Shrimps, Lobsters, and Crayfish. The body is 

 elongate, and the abdomen, which is greatly developed, ends 

 in a large fin-like tail-fan. They live in fresh water as well 



