The Emdence from Sexual Characters. 171 



viata Anglicay the animals are not hermaphrodites, as 

 earlier writers had supposed, but that the males, which 

 are rarely met with, are very much smaller than the 

 females. The latter sex is furnished with a digestive 

 tract which is quite complicated in structure, and is 

 armed at the mouth with a highly specialized masti- 

 cating apparatus. The digestive organs of the male, 

 on the other hand, are almost absent. The jaws, the 

 oesophagus and the mouth are wanting, and the stom- 

 ach and intestine are reduced to a f unctionless rudiment. 

 The males receive no nourishment after they leave the 

 egg, and they live only a short time. The presence of a 

 digestive tract is characteristic of all groups of animals 

 above the protozoa, so we are compelled to believe that 

 the ancestral form from which the Eotifera are de- 

 scended had, like the ordinary metazoa, a mouth, a 

 stomach, and an intestine; and no one who is at all 

 familiar with comparative anatomy can doubt that the 

 male, in which it is absent, rather than the female, in 

 which it is present, is the sex which has been modified. 

 The digestive tract is usually one of the first parts to be 

 developed in the embryo, and its disappearance or ab- 

 sence in the adult male rotifer is therefore very different 

 from the absence of the win2:s in certain female insects. 

 Wings appear very late in life, and the failure of the 

 female to acquire them is simply an arrest short of per- 

 fect development, while the absence of digestive organs 

 shows active degeneration. In 1855 Leydig verified 

 Dalrymple's observation (Zeit. f. Wiss. Zool. vi. p. 9G) 

 in the same species, and also in a second species of the 

 same genus; and as he was able to distinguish the out- 

 line of the male inside the egg, while this was still con- 

 tained within the body of the female, he removed all 

 reason for doubting that the two sexes belong to one 



