394 A HISTOEY OF RECENT CEUSTACEA 



outermost. They are attached pair by pair to the first five 

 segments of the peraeon as in the Cymothoidfe. Giarcl 

 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 aiathors 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. In the 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 Entoniscidae 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 Atlielgue 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 





