THE PELAGIC NEMERTEAN NECTONEMERTES 46] 
to nurse cells and are eventually absorbed by the one or two which 
mature. But these latter far exceed in size the eggs of the lit- 
toral species. 
There is thus a remarkable provision for insuring the develop- 
ment and fertilization of relatively few eggs provided with a 
large amount of food material. The studies of Brinkmann 
(17, ’17a) show that the period of reproduction may be very 
long, for there is no evidence of any influence of the seasons at 
the great depths at which these worms live. 
EXTERNAL ANATOMY OF N. MIRABILIS 
Turning now to the anatomical peculiarities of Nectonemertes, 
we find our material is such that we are enabled to supplement 
Biirger’s account (09) in certain details, as well as the very excel- 
lent descriptions which Cravens and Heath (’06) have published 
for N. pelagica, and Brinkmann (17, ’17a) for N. mirabilis, 
N. minima and N. primitiva. 
As shown in figures 1, 4, and 5, the shape of the long oval, 
flattened body is well adapted for a free-swimming existence in 
the intermediate depths of the ocean. In all cases where the 
precise locality is known the various species inhabit regions hay- 
ing a great depth of water, usually of a thousand meters or more. 
In most instances they have been taken in an open net or occa- 
sionally caught on fishing lines, in which eases there was no proof 
whether they came from the bottom or the surface or from any 
intermediate depth. The recent reports of Brinkmann (’17, ’17a), 
however, show conclusively that the species lives only at such 
great depths as have a temperature of 6° C. or less and a salinity 
not exceeding 35 per cent. In the North Atlantic Ocean these 
conditions rarely occur at less than 500 fathoms, and this is 
about the upper limit of the species. 
Size and shape. The body, as was well described by Verrill 
(92), is elongated elliptical, much flattened; with a pair of 
broad, thin lateral extensions of the body walls in a region from 
two-thirds to four-fifths the distance from head to posterior end 
of body, forming a pair of horizontal fins. The body is con- 
