THE PELAGIC NEMERTEAN NECTONEMERTES 463 
INTERNAL ANATOMY 
Body walls 
Integument. In all the specimens the epithelial covering of 
the body was almost entirely dislodged, leaving the ragged sur- 
face of the basement layer exposed. This loss of the body epi- 
thelium usually occurs in bathypelagic nemerteans during their 
capture. This is probably due in part to the change in pressure 
of the water while they are being brought from a depth of hun- 
dreds of fathoms to the surface and also in part to the great deli- 
cacy of the epithelial covering, making it easily removed in 
handling. What epithelium remains consists of a delicate layer 
of columnar cells, as described and figured by Cravens and 
Heath (’06) for N. pelagica. 
Basement layer. In distinction to the epithelium, the base- 
ment layer (figs. 6, 16, bm) is firm, but extremely thin as com- 
pared with that of littoral nemerteans. Its surface is thrown 
into irregular folds and corrugations for the attachment of the 
epithelium. 
Musculature. The body musculature is very much reduced, 
particularly along the lateral margins. Only in the region of 
the tentacles and in the caudal fin are the muscles on the sides 
of the body well developed. Elsewhere the circular layer consists 
of a thin sheet of tissue (figs. 6, 15, 16) directly beneath the base- 
ment layer. ‘The longitudinal muscles extend as broad sheets 
along the dorsal and ventral sides of the body, but almost dis- 
appear laterally (fig. 10). 
The longitudinal muscles are also much thinner along both 
median lines, but particularly on the dorsal side, directly above 
the proboscis sheath. In the caudal fin, on the contrary, they 
are well developed on all sides (figs. 8, 9). 
At intervals throughout the length of the body bundles of dor- 
soventral fibers connect the muscular layers of dorsal and ventral 
surfaces directly through the body parenchyma, and between the 
intestinal diverticula. ‘These dorsoventral bundles are likewise 
particularly highly developed in the caudal fin and in the tenta- 
cles (figs. 8, 9, 12, 13), as well as in the keel-like horizontal fins 
