HIGHER CRUSTACEA OF NEW YORK CITY 121 



If the second pair of maxillae be examined carefully, it will be 

 found that they are biramous, that is consist of a basal joint which 

 bears two rami or branches, known respectively as exopodite and 

 endopodite. This may be termed the typical form of a crustacean 

 limb, but it is often modified by the loss of one of the rami, the 

 exopodite. This is true of the mandibles and the first maxilla. 



The thoracic region, which follows and which, as mentioned 

 above, in the crayfish is fused with the head, bears eight pairs of 

 appendages. Of these the first three are comparatively small, are 

 biramous, function as accessory mouth parts and are known as 

 maxillipeds. The next five pairs are large with only a single branch 

 representing the endopodite and serve as the walking legs. The 

 first pair of them are largest and the penultimate joint is produced 

 distally to form a projection against which the ultimate joint bites, 

 thus forming a claw or chela. The next two pairs are chelate also, 

 while the last two are simple. 



To the bases of these appendages are attached long plumose hol- 

 low gills which are covered over and protected by the carapace. 

 Another set of gills is also attached to the inner wall of this chamber 

 directly above the preceding. 



The appendages of the third, fourth and fifth segments of the 

 abdomen are biramous and delicate and are used to aid in swimming, 

 and by the female as a place of attachment for the eggs. The sixth 

 pair are also biramous but are broad and flattened and together 

 with the last joint or telson (which is without appendages) form 

 a fanlike tail fin. With the aid of this and by suddenly bending 

 the body, the crayfish can give a swift leap backward through the 

 water. 



The appendages of the second abdominal appendage in the female, 

 and of the first and second of the male, are modified into genital 

 organs. 



Examine now another form of Crustacea, less familiar than the 

 crayfish or lobster, but exhibiting the crustacean characters in greater 

 simplicity : the common fresh-water amphipod, G a m m a r u s 

 fasciatus, found everywhere in fresh-water ponds. One of the 



