OP AETS AND SCIENCES. 95 



fibres, and an equally imperfect layer of longitudinal fibres. The 

 intestinal tract is divided into four distinct parts : a mouth cavity, a 

 pharynx, an ojsophagus, and an intestine. 



When closed, the mouth opening (PI. I. figs. 1, G), which is situated 

 on the ventral face of the first segment, has the shape of a brace (^-'^) 

 with its cusp directed forward. Numerous superficial grooves radiate 

 from it, the largest of them passing forward from the centre of the 

 o[)ening, the cusp, in some cases extending almost to the anterior 

 end of the pre-oral lobe. 



The mouth cavity (PI. II. fig. 11) extends from the mouth opening 

 vertically upward to a point just beneath tlie supra-oesophageal gan- 

 glion, where it joins the pharynx. Sometimes it is slightly convex 

 forward. Its walls consist of the internal cuticula, the layer of epithe- 

 lial cells, and the membrana propria. So far as observed, its epithe- 

 lial cells are not ciliate. 



The pharynx (PI. I. fig. 1 ; PI. II. figs. 11, 14, 15, IG) extends hori- 

 zontally backward from beneath the brain in the first segment to the 

 partition between the fifth and sixth segments, where it passes into 

 the oesophagus. Its greatest internal diameter occurs at about the 

 middle of the second (first post-oral) segment, where, measured verti- 

 cally, it is about three times that of the preceding and succeeding 

 parts of the alimentary canal. From this point it tapers in both direc- 

 tions, but more rapidly forward. Wlien seen in median longitudinal 

 section (PI. II. fig. 11) the cavity of the pharyn.x therefore appears 

 to be somewhat spindle-shaped, though that is far from being its true 

 form. This cavity is divided by two longitudinal folds, one projecting 

 from each lateral wall, into an upper and an under chamber (PI. II. 

 figs. 14, 15, IG), as described by Biilow ('83, p. 71) for Lumbriculus. 

 "When the pharynx is in its natural position a cross vsection (Fl. II. figs. 

 14, 1.5) shows that the upper chamber is triangular, with the vertex of 

 the triangle uppermost ; while the lower one is elliptical or crescent- 

 shaped with its long axis horizontal, and, when crescent-shaped, the 

 concavity of the crescent uppermost. Sometimes a peculiarity of the 

 lateral folds, attributable to the state of muscular contraction, causes 

 the apical part of the triangular upper space to project on each side, and 

 the whole upper chamber thus becomes T- or Y-shaped. The walls of 

 the upper space are composed of the internal cuticula, the membrana 

 propria, and a layer of much lengthened epithelial cells (PI. II. figs. 11, 

 16, et'.). the long axes of which are perpendicular to the surface of the 

 pharynx. These cells have elliptical granular nuclei, situated at their 

 deep ends, while their granular cell protoplasm is concentrated at their 



