500 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



structural and functional hermaphroditism ; in the 

 younger stages the testes are enormous as compared 

 with the ovary, and two penes are seen to be 

 developed ; the spermatozoa developed in the glands 

 of these Crustacea, with more highly differentiated 

 ancestry than the Cirripeds, are, it is said, motionless. 

 Later on, the testes diminish in size, and the ovarian 

 region comes into functional activity. In the Cryp- 

 toniscidse the male elements are matured during the 

 larval stage, and male free-swimming larvse copulate 

 with females of fixed habit and a less high degree of 

 organisation ; the male larvae subsequently become 

 degraded, and take on the characters and develop the 

 glands of the female (Kossmann). We have here to 

 do with what may be called phenomena of pro- 

 tandrous licrmaptiroditism. 



The differences between males and females are 

 especially well marked in many groups of Insects ; 

 as an ordinary rule the males are smaller than the 

 females, being, as it seems, developed more rapidly so 

 as to be ready to fertilise their often short-lived mate ; 

 where, on the other hand, the males fight with one 

 another, or carry the female through the air, they are 

 the larger of the two sexes. In many cases (Cicadas, 

 grasshoppers, etc.) the males are alone provided with 

 sound-producing organs, or, as so often happens 

 among butterflies, the males are much handsomer 

 in appearance ; sometimes, also, the females of one 

 species are of two distinct forms (dimorphic 

 females), and among the beetles we find that 

 males of one species may vary very greatly in the size 

 and character of their horns (Lucanidse). 



Differences in size obtain also among some 

 Arachnids ; the male spider, for example, being very 

 much smaller than the female, and often exceedingly 

 agile in escaping from her ferocity ; in the spider, as 

 in the crayfish, one of the appendages is modified to 



