SEXUAL DIMORPHISM IN NEMERTEANS. 37 



sex, but in general the exceptionally large individuals are females. 

 The third external sexual character, in which a real dimorphism 

 of the sexes occurs, has been found only in a single family living 

 in the deep sea in various parts of the world. And although 

 both males and females of one of these forms have been known 

 for many years the sexes have hitherto been placed in different 

 genera. The form to \vhich reference is made w r as described by 

 Verrill nearly thirty years ago (1892) as the type of a new family, 

 Nectonemertidae, the males being placed in the genus Nectone- 

 mertes. The species was named by him N. mirabilis. The most 

 remarkable feature of this new genus was the presence of a pair 

 of lateral appendages, or tentacles, on the sides of the body just 

 back of the head. 



A. SEXUAL DIMORPHISM IN NECTONEMERTES. 



Verrill correctly noted the fact that in N. mirabilis the smaller 

 and immature individuals had very short, or rudimentary, 

 tentacles while in the largest specimen these organs attained a 

 length much greater than the diameter of the body. The- four 

 specimens which Verrill had for study were collected by the 

 United States Fish Commission Steamer Albatross in the North 

 Atlantic Ocean in the region of the Sargasso Sea, between 37 

 and 41 degrees north latitude and between 66 and 73 degrees west 

 longitude. The depth of water at the collecting stations varied 

 from 600 to 1,700 fathoms. These specimens and one other 

 from the same locality have recently been fully studied by the 

 writer, and all prove to be males. 



The worms of this species have a remarkably fish-like appear- 

 ance (Figs. 1,2), with much flattened bodies, with horizontal 

 fins along the posterior half of the body, and a broad terminal tail 

 fin. They are obviously adapted to a pelagic existence, and 

 although there is no positive evidence as to the exact depth at 

 which this particular species lives, yet several other species of the 

 genus are know r n to live at intermediate depths in the deep oceans, 

 and other forms of a somewhat similar organization have been 

 taken in a closing net at a depth of several hundred fathoms. 

 The epithelium covering the body in all these forms is mostly 

 dislodged during capture, and in one or two species mentioned 



