298 



A HISTOEY OF KECENT CRUSTACEA 



PIG. 25. Diastylis Good- 



In the young and the males these plates are rudimen- 

 tary and without trace of setae. The inference, there- 

 fore, cannot be avoided that they have to 

 do with some maternal duty. ' It will be 

 found,' Professor Sars observes, ' that they 

 constantly have their place in the front 

 part of the marsupium, and with their 

 long setee work in among the eggs and 

 embryos contained in it. Their function 

 can, therefore, be nothing else than to 

 bring about the rotation or movement of 

 the eggs included in the marsupium, which 

 is very clearly observable in living speci- 

 mens, and which in the Mysidas and Iso- 

 poda is brought about in a somewhat dif- 

 ferent way, by a peculiar rhythmical 

 movement of the plates themselves of 

 which the marsupium is composed.' 



The third maxillipeds have a well- 

 developed exopod or swimming-branch, 

 ^he second joint to which it is attached 

 being almost always of very considerable size, and more 

 often than not produced at the outer apex. 



Of the five pairs of trunk-legs or perasopods the first is 

 usually much elongated. In almost all cases the second 

 joint in these limbs is the longest. It is only rarely that 

 the terminal joint is of the curved pattern found in the 

 so-called fingers of many Crustacea. The three hinder 

 pairs might, it is said, from the habits of the animals, 

 rather be called digging than walking feet. 



The pleopods of the male vary in pattern as well as in 

 number. They are only fully developed in the adult. 

 They usually consist of a two-jointed peduncle, and of this 

 the second and principal joint carries on the upper part of 

 its inner margin spines converted into coupling hooks. Of 

 the two branches the outer is two-jointed, the inner one- 

 jointed. Sometimes there is only a one-jointed outer 

 branch, and sometimes in place of well-developed pleopods 

 there are mere rudiments, or a group of seise. In the 



