THE PELAGIC NEMERTEAN NECTONEMERTES 459 
The evidence points strongly to the probability that one or 
more of these supposed species of Planktonemertes will eventu- 
ally prove to be the females of the species of Nectonemertes 
which are found in the same regions of the Atlantic; that is, 
between the Sargasso Sea and the Azores. 
Birger (09) has described the anatomy of one male specimen 
under the name N. mirabilis and a female which he identified as 
Hyalonemertes atlantica. Brinkmann (717 a) shows the former 
to belong to N. primitiva and the latter to N. minima. 
Still another species of Nectonemertes, N. japonica, has been 
described by Miss Foshay (’12) from six specimens collected near 
Misaki, Japan. All of these have well-developed tentacles and 
all are males. 
This makes a total of more than twenty specimens of Necto- 
nemertes known up to that time in which the sex is known and 
all are males. In Hyalonemertes, on the other hand, the few 
specimens studied are without cephalic gonads and are either 
known to be females or are presumably of that sex. But as the 
only apparent differences between Nectonemertes and Hyalone- 
mertes are the presence or absence of tentacles and the differ- 
ences in the sexual organs, and as the tentacles develop in the 
males only with the sexual maturity of the animals, the conclu- 
sion appears obvious that Nectonemertes is the sexually differ- 
entiated male of the various species, while Hyalonemertes and 
some of the specimens described as Planktonemertes represent 
the corresponding females. The species must all bear the generic ° 
name Nectonemertes, since this has priority.! 
This view was first advanced by Brinkmann (’12, 717)? who 
1. Positive confirmation of the sexual dimorphism of this form has more recently 
been furnished by Brinkmann (’17, ’17a), who has secured for study no less than 
116 specimens, including 32 males with tentacles, 6 young males, 63 females, and 
15 young, sexually undifferentiated individuals. 
2 The males form a regularly graded series in which the growth of the tenta- 
cles is perfectly correlated with the state of development of the gonads. In this 
series the tentacles range in size from the merest indications of lateral outgrowths 
of the body walls to the fully developed appendages two or three times as long 
as the width of the head. The females, on the other hand, show no trace of 
these appendages at any stage of life. Furthermore, a similar sexual dimorphism 
is shown by Brinkmann (717) to exist in two other species of the genus. in 
Balaenanemertes, however, a pair of very short tentacles is found in both sexes. 
