276 SEXUAL DIMORPHISM 



when a small stick of wood is presented to the male he seizes 

 it with these organs and holds on to it so firmly that he can 

 be lifted out of the water by the stick. This suggests that 

 the mandibles are used in fighting. Hesse, 1 on the other 

 hand, states that when a number of the animals were placed 

 in a jar of water, they bit one another indiscriminately, and 

 copulation was not observed. The females could not bite, so 

 that we may perhaps infer from the statement that the 

 males used their jaws on both females and other males, and 

 this suggests that under natural conditions the males fight. 

 In any case, whether the mandibles are used in fighting or 

 simply for seizing the female, we see a reason for their 

 presence in the male and absence in the female, in the 

 fact that the male uses them, while in the female they 

 have disappeared from disuse. It may be added that the 

 female vaginal orifice is said to be placed at the base of the 

 last pair of walking legs, and that the brood-pouch, according 

 to Dohrn, is not the cavity of the body, but is formed between 

 the cuticle and the body. In most other cases the brood- 

 pouch in Isopods is formed outside the body by flattened 

 plates belonging to the appendages. 



Now we have to consider the immature Gnathia or 

 larvae. These resemble the female, having narrow heads, 

 and the three posterior segments of the pereion fused and 

 distended. They have mandibles, two pairs of maxillae and 

 maxillipeds, but all these oral appendages are slender and 

 pointed, forming a piercing and sucking apparatus with which 

 they penetrate the skins of fishes, hanging on to them and 

 devouring their blood. They are not, however, permanently 

 attached. They are active swimmers, and when the fish on 

 which they are feeding is captured they either let go their 

 hold and escape in the water, or drop off soon after the fish 



1 "Crustaces Rares ou Nouveaux des Cotes de la France" Ann. Sci. 

 Naturelles, tome xix., 1874. 



