SEXUAL DIMORPHISM IN NEMERTEANS. 47 



The female of this species has about 25 pairs of typical inter- 

 diverticular ovaries. 



4. Paradinonemertes. In Paradinonemertes drygalaskii Brink- 

 mann ('16), of which two specimens are thus far known, the 

 spermaries have suffered a still further reduction in number, 

 being limited to two pairs of gonads situated immediately behind 

 the brain and between the stomach and the lateral nerve cords 

 (Fig. 17, sp). This is the extreme limit of reduction at present 

 known among the nemerteans. These two pairs occupy the 

 same position in the body as do the two anterior pairs in Plank- 

 tonemertes, and are apparently homologous with them. The 

 female of this species is still unknown. 



5. Pelagonemertes. It is in the genus Pelagonemertes that the 

 spermaries show the greatest deviation from the primitive ar- 

 rangement. In P. brinkmanni, a new species collected in the 

 northwest Pacific Ocean, there are five to seven egg-shaped 

 spermaries in a single cluster (Figs. 5, 7, sp.) on each side of the 

 head in front of the brain. These open by a group of small 

 genital papillae situated ventrally on the antero-lateral border 

 of each side of the head (Fig. 7, gp.}. 



In some cases the contraction of the tissues during the capture 

 and preservation of the worms has ruptured the delicate cephalic 

 w r alls and forced the spermaries quite outside of the body. 

 They then appear as if they were external appendages on the 

 anterior margins of the head. 



The ovaries likewise suffer great reduction in number as 

 compared with the forms previously considered, there being 

 usually only six pairs of these gonads (Fig. 6, ov). Occasionally 

 the number may be increased to seven or eight on one or both 

 sides. The immature ovary contains as many as four to six or 

 more small eggs, but as development proceeds most of these are 

 engulfed as food material for the one, two, or occasionally three 

 very large ova which reach maturity. The total number of eggs 

 which one of these worms produces in a season is therefore hardly 

 more than twenty to thirty. 



In Pelagonmertes rollestoni Burger ('09) finds likewise five or 

 six pairs of spermaries (Fig. 12, sp) closely grouped on each side 

 in front of the brain. The sperm ducts leading from each group 



