642 F. F. LAIDLAW. 



which the lower part of the pharyux projects (fig. 3, 

 Ph. T., Ph. Ti.), so tliat the roof of the chamber is formed by 

 the pharynx itself. The pouch agrees very closely with that 

 figui-ed by von Grraff [2] for Mesostomum Ehreubergii, 

 andj as in the latter, it is provided with a few muscle-fibres 

 attached to the walls at its widest part, running to the 

 body-wall. Neither the epithelium lining the wider part of 

 the pouch nor that on the exposed part of the pharynx is 

 ciliated, the cilia only extend to the narrow part of the 

 pouch. With this should be compared the condition found 

 in Mesostomum Ehrenbergii, v. Graff, in which the pouch 

 is ciliated throughout. In Mesostomum Cuenoti, recently 

 described by Dorler [5], on the other hand, only the roof of 

 the pouch, i.e. the projecting wall of the pharynx, is ciliated. 

 The pharynx itself has its principal axis a little elongated, 

 and running nearly dorsiventrally. It is pyriform, its narrow 

 end being ventral and the broader dorsal. The larger size 

 of the dorsal part is due to the greater size in that region 

 of the pharyngeal cells (fig. 3, Ph. C). 



The pharynx conforms to the type called by von Graff' 

 " pharynx rosulatus," characterised by the numerous large 

 gland cells which in sections appear as coarsely granular 

 cells, and also by the arrangement of the muscle-fibres. 

 The latter consist of a double outer layer, forming, as it 

 were, a muscular capsule, and a double inner layer lying 

 immediately under the epithelium lining the lumen of the 

 pharynx. Between these two definite layers run numerous 

 radial fibres (fig. 3, B.M.). The outer layer consists first 

 of longitudinal fibres {O.L.), — that is to say, of fibres running 

 parallel to the principal axis of the pharynx ; and immedi- 

 ately below these, of stouter circular fibres (0. C*V.). The 

 inner layer consists of a number of fine circular fibres 

 (fig. 4, I. c), lying immediately on the outside of the epi- 

 thelial lining of the lumen {Ph. P.), and outside these of 

 longitudinal fibres (fig. 3, I. L.), which like the inner circular 

 fibres are but feebly developed. The radial muscles are 

 most numerous in the ventral part of the pharyux. 



