HK^IKR CRUSTACKA OF NEW YORK CITY I23 



poda may or may not bear a jointed appendage known as a palp 

 [M. p]. This is frequently of value in determining- the species, and 

 when present, either it appears projecting as in the figure, or, fre- 

 quently in alcoholic specimens, the end of it may be seen between 

 the bases of the antennulae. 



As only one thoracic segment is fused with the head, there is but 

 one maxilliped [M.v. p], which is small. 



The seven following- segments constitute the thorax or pereion 

 [Per]. Its first two pairs of appendages, corresponding to the 

 second and third maxillipeds of the crayfish, are large and known 

 as gnathopoda [Gii]. They consist, as do the following five pairs 

 of legs, of six joints attached to a base plate, the coxa [C 1-7]. 

 The six joints beginning at the proximal end are named basus [B], 

 ischium [/], merus [j\I], carpus [Ca], propodus [P] and dactylus 

 [D], The ultimate joint, the dactylus, is bent back on the propodus 

 so as to form a sort of chela, or, as it is termed, a subchela. 



The next five pairs of legs are the pereiopoda. The first two are 

 of moderate size and the last three are larger and adapted for leaping 

 or walking. 



The last six segments are the abdomen or pleon [PI] . The three 

 anterior of them are larger than the rest and bear delicate biramous 

 appendages, the pleopoda [PI. d], which function as swimming 

 organs. The last three appendages are the uropoda [Ur] and are 

 biramous, stout and assist in leaping. 



The last pair of them, known as the terminal uropoda, differ fre- 

 quently in other species, from the preceding pairs and afford valuable 

 diagnostic characters. The terminal segment of the body, the 

 telson [T], as in the crayfish, has no appendages and is small. It 

 also varies greatly in the dift'erent species. 



The breathing organs of this group are in the form of lamellar 

 sacs attached to the bases of the pereiopoda. 



The eggs are carried in a brood pouch formed of plates attached 

 to the pereion of the female. 



Comparing now the two forms, we see that, in spite of their dis- 

 similar appearance, they are composed of the same elements, though 

 the elements may be greatly modified for different purposes. These 



