374 Mr. C. Chilton on Polymorphism 



The plates into which the integument is produced on the 

 outer sides of the basos and ischios interlap^ and appear thus 

 to strengthen the joints, which would otherwise be weak for 

 so long a limb, and they also serve to keep the carpus firmly 

 in its place when that joint is laid back upon the basos, as it 

 usually is when the limb is not being used. The tooth on 

 the anterior margin of the basos seems to be of use for the 

 same purpose. Spence Bate speaks of it as "an apparently 

 useless tooth " *. 



In Microdeuteropus niaculatus ^ , Chilton (see PI. X. fig. 1), 

 the first gnathopod has the basos long, expanding slightly 

 distally ; ischios nearly twice as long as broad, both joints 

 being almost free from setse ; the meros is narrow and is 

 produced inferiorly into a long acute spine reaching beyond 

 the end of the carpus ; it is slightly curved towards the extre- 

 mity and bears on its inferior or posterior margin a small tuft 

 of seta3 at about one third of the length of the joint from its 

 extremity ; the carpus is very large, more than twice as long 

 as broad, and bears a very few short setge at the distal end ; 

 the propodos is rather more than half the length of the carpus, 

 and is much narrower, and becomes narrower towards the 

 distal end ; inferior margin very slightly concave and thickly 

 fringed with setas more or less regularly arranged in tufts ; 

 there is also a tuft on the anterior margin at the base of the 

 dactylos ; dactylos more than half as long as the propodos, 

 narrowing regularly towards the extremity, where there are 

 a few short setee on the inner margin. 



In general shape the first gnathopod of Microdeuteropus 

 Mortonij Haswell (see PI. X. fig. 4), closely resembles that 

 of M. maculatus ^ , Chilton, but the setas are very different, 

 and there are also a few other small differences : the anterior 

 edge of the basos bears a thick fringe of rather long set^e ; 

 these setge are sparsely plumose towards the distal ends ; 

 similar setse are found on the anterior edge of the ischios and 

 on both lateral borders of the meros, which is hollowed out 

 anteriorly to receive the carpus; the setffi on the meros are 

 more or less regularly arranged in tufts, but the acute- pointed 

 end of the joint is naked ; ordinary simple set« are found on 

 the anterior margin of the carpus and on both margins of the 

 propodos ; the dactylos is very long, fully as long as the pro- 

 podos, and bears three or four tufts of sette on its concave 

 margin. 



It is evident from what I have already said that the genera 



* ' British Sessile-eyed Crustacea,' i., IiUroduction, p. li. 



