394 A HISTORY OF RECENT CRUSTACEA 
outermost. They are attached pair by pair to the first five 
segments of the peraon as in the Cymothoide. Giard 
and Bonnier regard them as representing parts of the 
exopods of the limbs. This seems to require what they 
admit to be a very hypothetical explanation of an Isopod’s 
leg, namely, that the fourth and fifth joints are the fifth 
subdivided, and that the small first joint is a fusion of 
the first and second. By this redistribution the long 
second joint becomes the third, and thus matches the long 
third joint so frequently found in the third maxillipeds of 
the higher Crustacea. But convenient as the hypothesis 
may be for attaining this piece of symmetry, other grounds 
for it are not as yet forthcoming. However that may be, 
the authors show that the structure of these marsupial 
plates admits of the view that they are to a considerable 
extent branchial, that is, assist in the oxygenation of the 
blood. The first pair has a special structure and func- 
tion. As examined in the genus Cancricepon the larger 
and upper member of the pair is found to be divided into 
two portions by a median fold with an outer crest. The 
front part covers the base of the maxillipeds, the lower 
part is covered by the opposite plate and floats freely in the 
marsupial cavity. There are two movements affecting this 
apparatus, one that alternately lifts and lowers it as a 
whole, the other alternately lifting and lowering the front 
and back. By this means a current of water is maintained 
both to the marsupial plates and to the embryos within 
them. Inthe podophthalmous host the water enters at the 
back and leaves by the front of the branchial cavity, so that 
in the parasite which lies with its head towards the tail of 
its host it naturally enters by the upper part and leaves by 
the lower. In the Entoniscide the arrangement is modi- 
fied to correspond with their position among the viscera. 
In regard to the chance of procuring specimens, the 
authors note that, to obtain ‘the Cryptoniscian stage’ and 
the young female of Athelque paguri, the pleon of Paguri 
should be carefully examined in the month of September, 
and that these interesting forms will be found much less 
rare than is often supposed. Dr. Hoek records that on 
