STKUCTUKE OF THE ISOPODA 317 



elsewhere almost of the same size as these. But the ex- 

 ceptions to this statement are exceedingly numerous. 

 Occasionally the first pair are opercular ; often they have 

 the form of gnathopods, that is to say. they are prehensile 

 instead of ambulatory. Sometimes they are weak and 

 slender, and agree in character with the three following 

 pairs, at others they agree with only the next two pairs. 

 Earely the last three pairs are adapted for swimming in- 

 stead of walking. The comparative lengths of the joints 

 are also subject to much variation, although the second 

 joint is scarcely^ ever shorter than the first or the third, the 

 seventh pair of legs in Phreatoicus typicus, Chilton, and 

 the first pair of the male in one or more species of Munna, 

 offering perhaps rare exceptions. Marsupial plates are de- 

 veloped in the females in varying number. The pedun- 

 cles of the pleopods often carry coupling spines. The two 

 first pairs of the pleopods are often modified in the male 

 as sexual organs. Usually the inner branch in some of 

 the pairs is branchial. All the pairs are sometimes want- 

 ing. The uropods vary greatly both as to shape, function, 

 and their position on the sixth segment. The liver is said 

 to consist of four or six backward-directed tubes. The 

 heart is generally but not always elongate, and usually in 

 part situated in the pleon. The ovaries and testes are with- 

 out a median section. The young usually quit the brood- 

 pouch with the last peraeopods still undeveloped. 



While in the Cumacea the pleon is in general very 

 considerably narrower than the trunk, in the Isopoda this 

 is only rarely the case, and sometimes the pleon even ex- 

 ceeds the trunk in breadth. 



Definition of ihe Sub-order Isopoda. 



The cephalon or carapace with rare exceptions leaves 

 free seven segments of the trunk. There are no branchial 

 sacs in connection with the first maxillipeds, although these 

 occasionally assist the respiration. Usually some of the 

 pleopods have a branchial character. None of the last five 

 pairs of trunk-limbs have natatory exopods. The first 



