546 H. LYSTER JAMESON, 



Here it is that the alteration in the relations of the fibres takes place, 

 here the ciliated groove first appears, and here in Echiurus and Thalas- 

 sema neptuni the vascular ring embraces the digestive tube. Here 

 also we find a sudden and very marked change in the nature of the epi- 

 thelium. Owing to the constriction , in preserved specimens at any 

 rate, the lumen is generally almost obliterated. For simplicity I shall 

 adopt the name "pre-intestinal constriction" for this point, a term 

 that has the advantage over such simple words as "Cardia" or "Pylorus" 

 in that it is not so likely to be interpreted as posessing a morpho- 

 logical significance (Fig. 12, p). 



The pharynx (Fig. 11 ph^ Fig. 14) is the first division of the 

 fore gut, extending for 4 or 5 mm from the mouth backwards. 

 Posteriorly it goes over gradually into the oesophagus, from which 

 to the naked eye it is indistinguishable except for its slightly dilated 

 appearance. The mouth closes with a thickened bundle of circular 

 muscles which forms a sphincter. The course of the pharynx is 

 straight from the mouth backwards. As already remarked by Rietsch 

 it lacks the external longitudinal musculature of the remaining parts 

 (Fig. 14). As we follow it back towards the oesophagus the longitudinal 

 fibres begin to appear outside the circular ones, and are plainly 

 visible about 5 mm behind the mouth. The circular musculature 

 (Fig. 14 c. m) is fairly thick, measuring | the thickness of the epi- 

 thelium at times. The epithelium consists of short columnar ciliated 

 cells with a few gland cells {gl). These glands become more numerous as 

 we pass over into the oesophagus, in correlation with which the basal 

 ends of the columnar cells become compressed and more thread-like. 

 The epithelium may be more or less folded according to the degree 

 of muscular contraction. It is needless to say that the outer surface 

 of the pharynx like the remainder of the gut is covered with peri- 

 toneum. 



The oesophagus (Figs. 11, 12 oe, Fig. 15), which is in pre- 

 served specimens about 25 — 30 mm long, forms a loop to the right 

 returning dorsally upon itself (Figs. 11, 28). This loop is held 

 firmly in its place by dorsal and ventral mesenteries, to be described 

 later. In Fig. 11 the mesenteries are dissected away. After returning 

 to the left side and coursing for a short distance backwards to the 

 left of the nerve chord, the oesophagus goes suddenly over into the 

 gizzard (Figs. 11 and 12 g). Shortly before this point the ventral 

 mesentery leaves the oesophagus. 



At its origin the oesophagus is only distinguishable from the 



