104 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



the offspring may be discharged by the male, e.g., Alytes and the 

 Lophobranchia. Male birds also often share with the female the 

 labour of building the nest, of bringing up and protecting the young. 

 But it is a rare exception to find, as in Cottus and the Stickleback 

 (Gasterosteus), that the care and protection of the young fall 

 exclusively upon the male, that he only bears the brood pouch and 

 alone builds the nest, an exception which bears strong witness to 

 the fact that the sexual differences both in form and function were 

 first acquired by adaptation. 



In extreme cases, the sexual dimorphism may lead to so great a 

 difference in the sexes that without a knowledge of their development 



Sp 



a 



FIG. 98. Chondracanthus gibbosus, magnified about 6 times, a, female from the side. 6, 

 female from the ventral surface with the male (F) attached, c, male isolated, under strong 

 magnification. An', anterior antenna ; An", clasping antennas ; F' and F", the two pairs 

 of feet ; A, eye ; OD, egg sacs ; Oe, oesophagus ; D, intestine ; M, mouth parts ; T, testis ; 

 Vd, vas deferens ; Sp, spermatophore. 



and sexual relations, the one sex would be placed in a different family 

 and genus to the other. Such extremes are found in the Rotifera 

 and parasitic Copepoda (Chondracanthus, Lernreopoda, fig. 98, , 

 b, c), and are to be explained as the result of a parasitic mode of life. 

 The difference in the two kinds of individuals representing and 

 maintaining the species, whose copulation and mutual action was 

 known long before it was possible to give a correct account of the 

 roal nature of reproduction, has led to the designation " sexes," from 

 which tho term sexual has been taken to apply to the organs and 

 manner of reproduction. 



