112 ^Y. F. R. WELDON. 



of three kinds ; the most numerous (fig. 3, gr ) are columnar, 

 staining moderately deeply, and crowded with granules ; 

 wedged in between these are certain cells, the peripheral ex- 

 tremities of which are conical (m. ep.), but which send inwards 

 fine processes, some of which are probably muscular, while 

 others are nervous. The cells of the third kind (fig. 3, x) are 

 pale, with deeply staining nuclei. Immediately below the 

 ectoderm, on the ventral side, is a delicate layer of transverse 

 muscles (r. m.), the fibres of which are, I believe, continuous 

 with many of the processes of the cells marked m. ep., though 

 this connection is not so easily seen in the head as it is in the 

 trunk (cf. fig. 10). 



The dorsal ectoderm of the head is composed of an indifi'er- 

 ent epithelium several cells thick (cf. fig. 3, where, however, 

 the curvature of the head has caused this portion to be cut 

 tangentially, so that the thickness of the ectoderm appears too 

 great). 



Passing backwards, the dorsal and lateral surfaces of the 

 body are uniformly covered, between the head and the anus, 

 with a more or less cylindrical epithelium, one cell thick (cf. 

 figs. 4, 5, 6, 8), which is ciliated only in the region of the trans- 

 verse rings already referred to. 



The ventral ectoderm, in the region of the mouth and lips, 

 is a simple columnar epithelium with narrow, elongarted cells 

 (figs. 4 — 6), but behind the mouth, on the whole ventral sur- 

 face of the trunk, it has much the same structure as on the 

 corresponding side of the head. The myo-epithelial cells, with 

 their processes, are, however, much better marked (figs. 8 and 

 10, m. ep.), and their connection with the circular muscles is 

 more easily seen (fig. 10), while the cells lying between them 

 are all of one kind — large, finely granular, and paler, with 

 rounded nuclei (figs. 8 and 10, gr.). In the figures the whole 

 of the conical ectoderm elements with processes are labelled 

 m. ep. ; it is, however, obviously probable that many are 

 nervous in nature. 



In the tail the ectoderm is throughout of the same charac- 

 ter as that on the ventral side of the trunk, except that the 



