MUSEUM OF COMPARATIVE ZOOLOGY. 143 



slightly enlarged, and blunt. The centre of the blunt surface shows the 

 terminal orifice, from which the eggs are being voided. 



The general external surface, more particularly on the lines and about 

 the rows of bristles, is often covered with minute algse 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 ventral nerve 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. 



1. Body 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 yu, on the side of the body, being 

 however perceptibly thicker (about 4 fx) over the median lines. On top 

 of the anterior chamber it is only 2 /a thick, which partially explains the 

 transparency of that region. At the front of the head, on the other 



