56 G 



COMPARA TIVE ANA TOMY 



CHAP. 



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

 diverticulum of the buccal cavity (J'ig. 456), and is preoral. Its 

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



Special. — In the proboscidal diverticulum, a narrower posterior neck and an 

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



J 10 



Fio. 4u(j.— Ptychodera minuta. The proboscidal, collar, and anterior branchial regions, cut 

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

 cidal diverticulum of the buccal cavity ; 2, ventral septum of the juoboscis ; 3, skeleton of the 

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

 oesophagus ; S, ridge forming the boundary between the ffisophagus and the branchial intestine ; 0, 

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

 with the gill-slits ; 12, ilorsal 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, cwlom of the collar traversed 

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

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

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

 cavity. 



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

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

 process, which runs through the proboscidal cavity almost axially. 



In Bahmoglossus canadensis, the neck is wanting, the head of the diverticulum 

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

 lumen may be interrupted here and there. 



