30 



broail , hori/outal ridge thai jiasisus tVuiii the .stoiiiach l<j the lett side of tlie 

 caecum between two large lobes of tlie caecum wlii(-h j)ro,iect forward above and 

 below the intestine ami stomach so as nearly to encircle the ventral end of the 

 latter. The caecum is seldom or never emi)ty but is usualh' partially distended 

 so that it extends about to the middle of the coelom and when in this condi- 

 tion it is pear shaped, the smaller end being dorsal. A small circular band of muscle 

 fibres divides the smaller end from the main portion of the sac. This band may 

 be strongly contracted .so as to produce a diaphragm-like curtain perforated by 

 a large central hole or it may relax so completely that the caecum becomes 

 narrowl}" oval. A number of folds are formed from the lining of the outer wall 

 of the ventral end of the caecum and of the adjacent portions of the upper and 

 lower walls. The folds are blade-like and radiate from the outer end of the crest 

 that separates the two ventral lobes of the caecum. The folds vary in size and 

 are arranged like the mesenteries of a sea anemone: that is, the largest folds are 

 equidistant from one anothei', smaller folds are placed midway between these, 

 still smaller folds are placed midway between each large and small fold , and so 

 on until the smallest folds are mere ridges. Each fold is corrugated by a series 

 of ridges which are parallel to the free edge of the fold and which alternate on 

 the opposite sides of the fold. Henri finds that the secretion of the caecal folds 

 assists pancreatic digestion. 



The wall of the caecum is formed of three coats : viz. the outer pavemental 

 epithelium of the peritoneum , a middle coat of mu.scle fibres , and a lining or 

 secretory epithelium. The latter presents three chief types which are connected 

 by transitional forms. 1) The epithelium covering the caecal folds and the adjacent 

 region is thick , densely ciliated , and palisadal. The nuclei lie at the base of the 

 cells and the distal portion of the cells is often filled with granular protoplasm. 2) 

 The epithelium of the greater part of the remainder of the ventral portion of 

 the caecum is exceedingly irregular but the characteristic cells are columnar or 

 club-shaped and usually occur in groups of three or more. The nucleus often lies at 

 the distal end of the cell , while in other cases the distal end of the cell is 

 vacuolated and the nucleus is at the base. The cells are sparingly ciliated. 3) The 

 epithelium of the dorsal end of the caecum is cubical or pavemental and ciliated. 

 It is possible that tension may change the epithelium from one type to another 

 but this is improbable because the three types often occur close together in a 

 portion of the wall which is apparently under uniform strain. 



The communication (Plate I Fig. 2) between the caecum and the neighboring 

 cavities of the gut is by means of a very remarkable five-way valve whose lumen 

 may be roughly compared to a letter M. The two upper points of the M-shaped 



