250 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Upon the crest of the head (see Lewis, '97, Plate 1, Figs. 2 and 3) 

 there are many sense organs of large size. 



With the first setigerous segment (Plate 8, Fig. G8) begins a well 

 marked grouping of the sense organs, some 15 to 20 in number, posterior 

 to and in the immediate vicinity of the ventral sette. A similar grouping 

 is shown in all the thoracic segments. At the anterior and posterior 

 margins of the thoracic segments the sense organs are more numerous, 

 forming ill defined circular bands, but in other portions of the segment 

 they are rather uniformly distributed, though without any distinct plan. 



b. In the Abdominal Region. 



It is more difficult to study the sense organs in the abdominal region, 

 on account of the extreme thinness of the cuticula ; but by careful ex- 

 amination with high powers such organs can always be found. Toward 

 the posterior extremity of the worm the cuticula again becomes thicker 

 and the sense organs more prominent. The last segment of the tail ends 

 in a funnel-shaped expansion within which numerous sense organs are 

 found. Such a conspicuous grouping of sense organs as is seen in the 

 thorax cannot be made out in the abdomen, although it seems evident 

 that here too there is a slight concentration of organs around the setae 

 and at the anterior and posterior margins of the segments. 



As a rule the sense organs vary in size directly with the thickness ot 

 the cuticula. • 



They resemble somewhat those described by various writers for Lumbri- 

 cus, but differ from them in the following respects, at least : (1) no sheath 

 cells are present ; (2) gland pores are not absent from the region imme- 

 diately surrounding them ; and (3) the number of cells composing an 

 organ is much smaller than in the case of Lumbricus. 



•o" 



4. Structure of Epidermis. 



The epidermis of both these annelids shows three types of cells : 

 (1) ordinary epidermal cells, (2) gland cells, which are very numerous in 

 certain regions, and (3) the cells of the sense organs. 



a. Ordinary Epidermal Cells. 



The ordinary epidermal cells (Plate 7, Figs. G3-65), the thread cells 

 of certain writers, show great variation of form in different regions of the 

 body. They are somewhat cylindrical, have an elongated nucleus and 

 possess usually two or more thread-like roots, as long as the rest of the 

 cell. The nucleus lies immediately above the point where the roots 



