THE PELAGIC NEMERTEAN NECTONEMERTES 467 
PROBOSCIS AND PROBOSCIS SHEATH 
Proboscis sheath. The opening of the rhynchodeum is on the 
dorsomedian part of the tip of the head (fig. 17). The rhyncho- 
deum (70) is very short, reaching less than half way to the brain, 
and is lined with highly columnar epithelium which, in some of 
the specimens, has become entirely dislodged. It is surrounded 
by interlacing circular and muscular fibers, which in the con- 
tracted state make a rather thick layer, but which doubtless be- 
comes very thin when the chamber is open for the passage of the 
proboscis. Radiating in all directions from the rhynchodeum 
are delicate muscle bundles, homologous with the dorsoventral 
muscles of the body, which hold this organ in its proper position 
in the parenchyma which surrounds it. At the point where the 
proboscis is attached these muscles become enormously increased. 
The proboscis sheath extends through about nine-tenths the 
length of the body, terminating just beyond the base of the caudal 
fin (fig. 1). 
The rhynchocoel varies greatly in diameter according to the 
state of contraction of the body. The difference in this respect 
between the voluminous cavity in the contracted specimen 
shown in section in figure 10 and the narrow tube in the well- 
extended specimen illustrated by figure 1 is very striking. 
The wall of the proboscis sheath remains thin and with but 
few circular muscles for some distance back of the brain, but 
gradually increases in thickness posteriorly (figs. 10,17). Except- 
ing In its anterior portion, where the proboscis sheath shows more 
or less distinct circular and longitudinal muscular layers, the 
musculature of this organ consists of interlaced circular and lon- 
gitudinal muscular fibers similar to those found in well-known 
species of Depanophoridae. 
Instead of terminating in the body musculature, as Verrill 
(92) erroneously states, the proboscis sheath gradually becomes 
extremely slender toward its posterior end and terminates in a 
great mass of body parenchyma, entirely free from even dorso- 
ventral muscles. 
Proboscis. The length of the proboscis nearly equals the length 
of the body when well extended (fig. 17), but in contracted speci- 
