272 



SEXUAL DIMORPHISM 



P. pulchellus, the immature as P. falcatus,. and the female as 

 P. pelagicus. 



Isopoda. Among the Iso^od^Ysim.Anthuridw, there is one 

 genus, JSisothistos, in which the difference between male and 

 female is very remarkable. Both are slender and elongated, 

 but the male is not only more slender, but is sinuous and 

 worm-like in shape, and it inhabits the tubes of Vermilia, 

 one of the Serpulidae, annelids which secrete a sinuous cal- 



Fig. 29. Podocerus falcatus (Mont.), a British Amphipod. A, male ; B, female. 



careous tube attached to stones, shells, or rocks. The whole 

 appearance of the male singularly resembles that of a small 

 Serpula, the operculum and branchiae being imitated by the 

 expanded posterior appendages and telson, and the hairs 

 representing the parapodial setae. Here the difference 

 between the sexes has nothing to do with sexual intercourse 

 or reproduction, but is only related to the different modes of 

 life. If it were a mere question of selection there is no 

 reason why the selected variations should not be transmitted 



