SECT, x REPRODUCTION 145 



limbs into the brood pouch formed by a modification 

 of these limbs. 



The external aperture of the genital ducts corre- 

 sponds well, as has already been pointed out, with one 

 of the apertures of the nephridia of the original Crus- 

 tacean-Annelid. This view, which is now generally 

 accepted among zoologists, is supported by the fact 

 that, among the higher Crustacea, the position of the 

 apertures is not always constant, i.e. they do not always 

 occur on the same segment or segments. Indeed, the 

 male and female apertures may occur on different 

 segments in the same species (e.g. in the Crayfish). 

 Wliiie there is variation in this point, there is con- 

 siderable constancy in the position of the openings 

 relatively to the ventral middle line of the body. 

 Both these points are important in homologising the 

 genital apertures of the Crustacea with the nephridial 

 apertures of the Annelida. 



Many if not all of the Apodidae are now, as was 

 before stated, hermaphrodite. The small oval sperm 

 cells form out of the epithelium of the genital tubes 

 in the manner illustrated (Fig. 33, /). In some species 1 

 the whole of the epithelium at the extreme posterior 

 end of the genital tube breaks up into sperm cells. 



The sperm cells of the Polychaeta are always (?) 

 thread-like, while those of Apus are round or oval : 

 this is no doubt a secondary modification. But 

 the round form of the sperm cells in Apus may 

 perhaps be the starting point for the many peculiar 

 shapes found among the higher Crustacea. Other 



1 L. glacialis and L. Spitzbergensis, see Fig. 32. 



L 



