SEXUAL DIMORPHISM IN NEMERTEANS. 4! 



but that they are specially adapted for grasping. The highly 

 developed horizontal and caudal fins show that the animals are 

 free swimming. When it is remembered further that the ten- 

 tacles reach their full size only at the time of sexual maturity, 

 the conclusion seems reasonable that they are then used for 

 grasping the female and clinging to her during the act of in- 

 semination, as Brinkmann ('12) has suggested. Associated with 

 these organs are spermaries with highly muscular walls (Fig. 8) 

 opening by sperm ducts leading to the ventral surface of the 

 head directly anterior to the tentacles (Figs. I, 8). The number 

 of such spermaries is limited to from twenty to thirty. The 

 female also differs from most littoral nemerteans, but agrees 

 with those of other bathypelagic species in having relatively few 

 pairs of ovaries along the sides of the body (Fig. 2). And each 

 ovary produces but one or two eggs of relatively enormous size 

 as compared with those of most littoral species. 



The reduction in the number of spermatozoa to a small 

 fraction of the number found in a worm of a littoral species of 

 similar size, and a corresponding reduction in the number of ova 

 produced by the female, has evidently necessitated a conservation 

 of the genital products by adaptations for securing the fertiliza- 

 tion of the maximum proportion of the eggs produced. 



Hence the advantage of pairing instincts and organs for ac- 

 complishing this result in place of the more primitive and waste- 

 ful processes which obtain in the littoral forms. These latte^, it 

 will be remembered, in many cases discharge an enormous 

 number of spermatozoa and minute eggs into the water about 

 them, with relatively small chances of any particular egg pro- 

 ducing an embryo. In some of the smaller forms, on the other 

 hand (as Amphiporus), there is a kind of pairing in which the 

 two sexually mature individuals of opposite sex place their 

 bodies side by side and the genital products of both atre dis- 

 charged together in a mass of mucus secreted by the integument 

 of the two worms. The littoral forms moreover live in a com- 

 paratively limited environment and thus have a great advantage 

 in this respect over these bathypelagic species, the habitat of 

 which may extend over hundreds of miles of ocean, with a vertical 

 range of several hundred fathoms. Hence, the advantage of 



