298 A HISTORY OF RECENT CRUSTACEA 
In the young and the males these plates are rudimen- 
tary and without trace of sete. 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 setae 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 Myside 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.’ 
F1G.25.—Diastylis Good- The third maxillipeds have a well- 
mari lipel of feeale developed exopod or swimming-branch, 
bans): the 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 perzeopods 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 sete. In the 
