48 WESLEY R. COE. 



open close together on the ventral surface of the head and close 

 beside the rhynchodeal opening, somewhat as described above 

 for P. brinkmanni. 



In Pelagonemertes mosleyi there are seven or eight pairs of 

 ovaries, while in P. rollestoni the number may sometimes be re- 

 duced to four or five (Fig. 13, ov). As each of these ovaries pro- 

 duces but one very large egg, or occasionally two or three, the total 

 number of eggs which mature in a season is hardly more than 

 ten to twenty under the most favorable conditions. This number 

 is in marked contrast to the hundred million which a single large 

 female of one of the littoral species discharges in the space of a 

 few days. The few however are so abundantly provided with 

 nourishment that it is probable that the entire embryonic de- 

 velopment is completed without the necessity of securing addi- 

 tional food. Since the oviducts are formed long before the 

 maturity of the ova, it seems reasonable to assume that fertiliza- 

 tion is internal, and it appears quite possible that the entire 

 embryonic history is completed within the parent's body. 



Summary. The foregoing brief survey of the structural mod- 

 ifications of the body, and especially of the organs concerned in 

 reproduction , which have recently been discovered in the pelagic 

 nemerteans shows that the series may be looked upon as represent- 

 ing possible steps in an evolutionary series leading from the 

 primitive condition of the well-known littoral forms to the highly 

 specialized adaptations of organisms living as a sparse popula- 

 tion in the vast areas of the open oceans. These modifications 

 all tend toward a reduction in the number of gametes produced, 

 correlated with increasing provisions for insuring fertilization and 

 the survival of the relatively few offspring produced, and leading 

 eventually to a true sexual dimorphism. 



BIBLIOGRAPHY. 

 Brinkmann, August. 



'12 Bathynectes murrayi n. gen. n. sp. Eine neue bathypelagische Nemertine 

 mit ausseren mannlichen Genitalien. Bergens Museums Aarbok, 1912, 

 no. 9. Pp. 1-9; pi. i. 



'16 Die pelagischen Nemertinen der deutschen Siidpolarexpedition 1901-1903 

 (Vorlaufige Mitteilung). Bergens Museums Aarbok, 1915-16', pp. 1-9, 

 pi. i. 



