TTTBIFEX. 323 



opens to the exterior on the last segment (PI. II, 

 fig. 4, in.). It is overlaid in the anterior segments by 

 the supra-intestinal blood vessel, and more posteriorly by 

 the dorsal vessel, and itself covers the ventral vessel 

 which lies freely in the body cavity. The intestine is 

 kept in place by the septa, which constrict it slightly at 

 intervals, and by special muscles which pass from it to 

 the body wall. Its lumen is larger than that of the 

 oesophagus, and is unfolded. It is ciliated throughout, 

 the cilia being specially long and abundant in the posterior 

 segments of the body, especially the last six or seven 

 segments. 



The structure of the intestinal wall from within 

 outwards is as follows : — 



(a) A single layer of somewhat elongated cells 

 forming the ciliated epithelium. The cilia are not very 

 conspicuous except at the anterior and posterior parts of 

 the intestine (PL III, fig. 13, ct.ep.). 



(b) A very thin muscular layer composed of a 

 number of isolated circular (on the inside) and longi- 

 tudinal (on the outside) muscle fibres (fig. 13, cm.; l.m.). 



The wall of the intestine is very vascular, and the 

 blood vessels are situated between the ciliated epithelium 

 and the layer of circular muscles (fig. 13, b.v.i.). These 

 vessels form what is known as the intestinal network (see 

 circulatory system). 



(c) A glandular layer composed of a very large 

 number of unicellular glands which cover the entire 

 surface of the anterior half of the intestine, and also 

 cover the dorsal blood vessel (fig. 13, c.c). It is to the 

 presence of these glands that the dark colour of the 

 intestinal wall is due. They were called chloragogen 

 cells by Claparede, and this name is still applied to them. 

 When examined in section, they are seen to be somewhat 



