566 



COMPARATIVE ANATOMY 



CHAP. 



reaching as far as the base of that organ. This is the proboseidal 

 divertieulum of the buccal cavity (Fig. 456), and is preoral. Its 

 epithelial wall is a continuation of the epithelial wall of that cavity. 



Special. In the proboseidal divertieulum. a narrower posterior neck and an 

 anterior head or body can usually be distinguished. In section, the neck appears 



FIG. 456. Ptychodera minuta. The proboseidal, collar, and anterior branchial regions, cut 

 through the middle line, and seen from the cut surface, diagrammatic (after Spengel). 1, Probos- 

 eidal divertieulum of the buccal cavity ; 2, ventral septum of the proboscis ; 3, skeleton of the 

 proboscis ; 4, buccal cavity ; 5, ventral vessel of the collar ; 6, ventral nerve cord of the trunk ; 7, 

 oesophagus ; 8, ridge forming the boundary between the oesophagus and the branchial intestine ; 1), 

 ventral blood vessel of the trunk ; 10, dorsal blood vessel of the trunk ; 11, branchial intestine, 

 with the gill-slits ; 12, dorsal nerve cord of the trunk ; 13, dorsal blood vessel of the collar ; 14, 

 roots of the collar cord ; 15, collar cord ; 16, proboscis pore ; 17, crelom of the collar traversed 

 by muscle fibres ; 18, anterior wall of the collar ; 19, nerve layer at the base of the proboscis ; 

 20, central blood vascular cavity of the proboscis ; 21, heart vesicle ; 22, proboseidal glomerulus ; 

 23, proboseidal epithelium ; 24, a part of the longitudinal musculature traversing the proboseidal 

 cavity. 



semilunar, with the concavity directed downwards. In Schizocardium and Glandi- 

 ceps, the head is continued anteriorly into a narrow blind canal, the vermiform 

 process, which runs through the proboseidal cavity almost axially. 



In Balanoglossus canadensis, the neck is wanting, the head of the divertieulum 

 consequently forming a constricted vesicle. In other species, the continuity of the 

 lunun may be interrupted here and there. 



