and Sexual Conditions in certain Isopoda. 419 



attention to rudimentary appendages upon the first two 

 abdominal segments in the female, which Schobl had ah-eadj 

 designated rudimentary male copulatory organs, an interpre- 

 tation with wliicli I entirely concur. 



It is a well-known fact that a remarkable sexual difference 

 occurs in Isopods with reference to the size of the adults. la 

 Pldtyarthrus the males appear considerably smaller than the 

 females. They exhibit on the whole characters belonging to 

 immature females, and this applies especially to the size of 

 the brain ; for in both sexes up to a certain stage the brain 

 grows at the same rate. Adult males have a brain of pre- 

 cisely the same size as that of immature females of about the 

 same dimensions. The female continues to grow, but the 

 brain undergoes no further increase in size, so that it finally 

 comes to lie in a large cephalic cavity ; while in the males, on 

 the contrary, the cephalic cavity is entirely filled by the 

 brain. It would be quite a plausible view to suppose that at 

 one time all individuals attained to male sexual maturity at a 

 stage at which they had not yet reached their definitive size. 

 After fulfilling their sexual function as males they continued 

 to grow and developed into sexually mature females. In the 

 stage of male sexual maturity the brain was very large; it 

 still entirely filled the cephalic cavity, while the oviducts were 

 present only in the form of blind invaginations of the hypo- 

 dermis, as is yet the case in sexually mature males at the 

 present time. 



A successive hermaphroditism of this kind, however, proved 

 to be not advantageous, and in consequence of this there 

 ensued a separation of the sexes. Certain individuals 

 remained stationary at the stage of male sexual maturity, in 

 consequence of which they remind us of immature indi- 

 viduals : the rudiments of the oviducts, too, have persisted in 

 them. Other individuals grew on directly into females, since 

 in these there has been a ca^nogenetic cessation of the appear- 

 ance of male sexual organs. As remnants of a male maturity 

 which formerly appeared in them these individuals possess 

 rudiments of male copulatory organs. 



In Platyarthrus accordingly proterandrous hermaphro- 

 ditism at one time occurred. In other Isopods it may perhaps 

 have been proterogynous. This was the case in Sphceroma 

 rugicauda^ for instance, in the internal sexual organs of which 

 Leichmann discovered what were undoubtedly hermaphrodite 

 rudiments. This author observes also that all young speci- 

 mens exhibit the general appearance of females, which in the 

 case of certain individuals is not exchanged for the detinitive 

 masculine form until shortly before the attainment of male 



