137 



lining of that part of the digestive tract in structure and staining re- 

 action. The extremities are characterized by the disappearance of all 

 traces of cell walls and nuclei, thus presenting a somewhat homogene- 

 ous appearance. The entire surface is covered by cuticula. Each or- 

 gan is provided with a muscle which extends ventro-caudad from its 

 base to the body wall. 



Intestinal Diverticula. — The intestinal diverticula comprise two 

 lateral sac-like evaginations which arise from the intestine in the pos- 

 terior part of VIII. They extend cephalad from the point of origin 

 and fill the greater part of the coelom in that region. Their dimen- 

 sions increase from the point of origin towards the anterior part of 

 VIII. They are somewhat flattened laterally, the greatest diameter 

 being in a dorso-ventral direction. The structure of these organs 

 (PI. XII, Fig. 59) is very interesting. Each diverticulum contains 

 a large central cavity, a continuation of the lumen of the intestine, 

 which communicates with it by a single, dorso-lateral, slit-like open- 

 ing. Another of the characteristic features of the structure of this 

 organ is the intricate folding of the inner lining. The dorsal portion 

 of the side adjacent to the alimentary canal shows but little if any 

 folding, but the entire opposite wall is conspicuously folded. This 

 folding, which involves most of the thickness of the wall, occurs all 

 along the ectal and ventral sides. Examination with high magnifica- 

 tion shows a series of blood sinuses which are intimately related to 

 the walls of the diverticula. In the anterior part of each organ they 

 are inconspicuous, being confined to small spaces in the walls and folds. 

 Towards the intestinal connection, however, they become more appar- 

 ent, and occupy considerable space between the parts of the walls as 

 well as the numerous spaces in the folds, as indicated in Figure 59. 

 The spaces in the folds appear slit-like in transverse sections. The 

 sinuses increase in size and diminish in numbers posteriorly until a 

 few large sinuses result from the union of the smaller ones. Ulti- 

 mately one very large sinus appears, which is the main channel of 

 connection with the perivisceral sinus at the junction of the divertic- 

 ulum with the intestine. The ectal surface of the diverticulum is to 

 some extent covered with chloragog cells. The ental surface shows 

 no such cells, but is covered with a peritoneal layer of the usual type. 

 The wall of the diverticulum is composed of (i) an external peri- 

 toneum, either modified into chloragog cells or of the usual type, (2) 

 a middle region, occupied by blood sinuses, and (3) an inner, rather 

 thick, greatly folded, non-ciliated epithelium. Slightly cephalad to the 

 junction of the diverticulum with the intestine, however, this epithe- 

 lium begins to take on the appearance of the lining epithelium of the 



