SEXUAL DIMORPHISM IN NEMERTEANS. 43 



B. SEXUAL DIFFERENTIATION IN OTHER BATHYPELAGIC 



NEMERTEANS. 



The distinction between sexual differentiation and sexual 

 dimorphism should closely be borne in mind. As commonly 

 used the former term has reference to those characteristics 

 which distinguish the sexes in a form in which both sexes are 

 easily recognized as belonging to the same species, while the 

 latter implies morphological features so profound that the two 

 sexes might appear to belong to different species or even larger 

 groups. Among the nemerteans known at the present time only 

 Nectonemertes, and possibly Bal<znanemertes, belong to the 

 latter category. 



It may be worth while to consider in this connection a few of 

 the many cases of sexual differentiation which lead toward the 

 true sexual dimorphism described above for Nectonemertes. In 

 many species of nemerteans there can be no question as to sexual 

 differentiation, for all individuals are hermaphroditic and to 

 some extent protandric, as in the fresh-water Stichostemma and 

 the terrestrial Geonemertes (Coe, '04). Here the young and 

 small individuals function as males. A period of growth com- 

 monly follows the discharge of most of their spermatozoa, when 

 they assume the characteristics of the female. In some cases 

 both spermatozoa and ova develop at the same time. Some of 

 these forms are viviparous, but there are also several cases of 

 viviparity known among littoral species which are neither her- 

 maphroditic nor protandric. 



The primitive condition of the reproductive organs in the 

 nemerteans, as is well known, is that of separate sexes, in both 

 of which the gonads occupy similar positions along each side of 

 the body throughout almost its entire length back of the head- 

 The gonads are usually in pairs, alternating regularly with the 

 intestinal lobes, or diverticula. The number of such gonads 

 may be many thousands in a large worm, and each ovary as 

 well as each spermary produces a large number of gametes. 

 It is estimated that a large female Cerebratulus, which may 

 reach a length of two meters or more, produces upwards of one 

 hundred millions of eggs in one season. These are all discharged 

 within the space of a few days. And in some littoral species no 



