158 W. B. BENHAM. 



sections a small pit is seen at the apex (PI. XX, fig. 12). A 

 prostomial pore is present in some aquatic Oligochsetes, e. g. 

 the Enchytrajidse, and Vejdovsky figures one for Criodrilus,i 

 but both Rosa2 and Collin^ deny its presence ; and I have been 

 unable to find it on a re-examination of my sections of that 

 worm. 



The pygidium (fig. 3) is a large truncated conical somite 

 carrying the dorsal anus at its end ; the two prsepygidial somites 

 are much shorter than the preceding somite, and these three 

 together are about equal in length to the pygidium. In the 

 majority of specimens the " tail " was regenerated, as is so 

 frequently the case in Criodrilus, Pontoscolex ( = Uro- 

 chseta), Lumbriculus, &c. 



The chaetae have the usual Lumbricid shape and arrange- 

 ment in two couples on each side ; the distal moiety of the 

 chaeta presents three or four transverse irregular ridges (fig. 5), 

 as in Rhinodrilus and several allied forms. In those cases 

 where the chsetse are unworn the point is rather sharply 

 hooked. There is no diflference in size and shape between the 

 ordinary and the clitellar chsetse. 



§ The Minute Structure of the Integument. 

 The general structure of the epidermis is similar to that of 

 Lumbricus, &c. The clitellum (fig. 30) is several cells 

 deep, or, more correctly, the club-shaped cells are of several 

 different lengths, some reaching to the circular layer of mus- 

 cles, others not much further than to the base of the epidermal 

 cells. The clitellum is some ten to twelve times thicker than 

 the ordinary epidermis ; and on each side of the ventral surface, 

 just outside the ventral chsetse, becomes still thicker, forming 

 in a transverse section a rounded prominence (fig. 19). This 

 is the tubercula pubertatis, consisting, as I have figured 

 for Microchseta,^ of very long cells, somewhat like the 



» • System und Morphol, d. Oligocliaeten.' 



' " Sul Criodrilus lacuum," 'Mem. d. Reale Acad. d. Sc,' Torino, 37. 



^ • Zeit. f. wiss. Zool.,' 40. 



< • Proc. Zool. Soc.,' 1892. 



