SEXUAL DIMORPHISM IN NEMERTEANS. 



WESLEY R. COE, 

 OSBORN ZOOLOGICAL LABORATORY, YALE UNIVERSITY. 



It has been generally assumed by zoologists that the sexes of 

 the nemerteans are indistinguishable, with the exception of 

 possible size and color differences, without an examination of 

 the gonads. While this is generally true certain facts have been 

 recently brought to light which show, as will be explained below, 

 that in at least one genus the sexes are truly dimorphic. The 

 male in this case not only bears the gonads in an entirely different 

 part of the body than does the female, but is also distinguished 

 by the possession of a pair of large lateral appendages which are 

 apparently used as copulatory organs. The only other instance 

 of an external appendage in nemerteans is the caudal cirrus in 

 Micrura and related genera, but this is without relation to sex. 



Many cases are known in which both size and color differences 

 distinguish the sexes when mature. Thus it has long been 

 known that in the common Cerebratulus lacteus Ver. the males 

 in the breeding season (Coe, '95) are suffused with bright red 

 in the anterior portions of the body and deep red in the intestinal 

 region where the spermaries are located, while the females are 

 much darker, duller red or brownish red, with a grayish tinge 

 except in the anterior portions of the body. This differentiation 

 in color is apparent some two or three months before the sexual 

 products are ready to be discharged, in April, although in the 

 summer and autumn months both sexes are nearly alike, with 

 pinkish white bodies and yellowish or brownish intestinal di- 

 verticula. Many other species show somewhat similar differ- 

 ences in color when sexually mature, or in the breeding season 

 of those which live for several years. It seems not improbable 

 that the sexes in every species if carefully observed would likewise 

 be recognizable by color modifications when their sexual products 

 are ripe. 



Little evidence is available as to the size factor in relation to 



36 



