MUSEUM OF COMPARATIVE ZOOLOGY. 143 
slightly enlarged, and blunt. The centre of the blunt surface shows the 
terminal orifice, from which the eges are being voided. 
The general external surface, more particularly on the lines and about 
the rows of bristles, is often covered with-minute algæ and dirt. A 
similar mass frequently envelops the posterior end, making the deter- 
mination of its character a difficult matter, especially in the case of the 
female, if an egg mass be protruding from the opening. It may have 
been such an appearance which caused Verrill (79, p. 187) to describe 
the female as possessing a small terminal papilla. 
2. INTERNAL. 
The general plan of the internal anatomy may be easily understood 
from any cross section (Fig. 11). Passing from the surface inward, the 
thick cuticula is followed by a thin hypodermis, beneath which is the 
highly refractive muscular layer. The protoplasmic ends of the muscle 
cells, together with other elements, form the layer immediately surround- 
ing the body cavity. Dorsally and ventrally the muscular layer is 
broadly interrupted by the prominent median lines. The body cavity 
contains in a varying position the alimentary canal, which is strikingly 
small and sometimes wanting. A sac-like organ varying in size hangs 
from the dorsal line into the body cavity, which may also be filled with 
generative products. The ventral line encloses the ventra! serve cord. 
A comparison of the various figures will show the variation in the pro- 
portions of the ventral line at different points in the body and in the 
two sexes. 
V. Anatomy and Histology. 
l. Bopy WALL. 
In treating of the finer anatomy I shall consider first the structure 
of the part under consideration in the male only, and at the end of each 
section give a comparative account of its character in the female. 
a. Cuticula. 
The cuticula, which covers the entire body and is continuous with the 
lining of the oesophagus and of the terminal sexual opening, is of nearly 
equal thickness throughout, averaging 3 u on the side of the body, being 
however perceptibly thicker (about 4 u) over the median lines. On top 
of the anterior chamber it is only 2 u thick, which partially explains the 
transparency of that region, At the front of the head, on the other 
