INTERNAL ANATOMY 247 



The ventral wall is formed of elongated gland cells, and in the thickest part, at the base 

 of the gland cells, there is a thin transparent layer, which looks like nerve tissue. The 

 ventral wall of the anterior lobe of the proboscis is composed of a thin layer of epidermal 

 cells, basement membrane, and coelomic epithelium, but in C. densus this region is 

 comparatively thick and the epidermal cells have vacuoles like the refringent vesicles 

 in the end-bulbs of C. dodecalophus (PI. XXXVI, fig. 2). Behind the attachment of the 

 proboscis to the collar, the dorsal and ventral walls approach each other, diminishing the 

 proboscis cavity. Ultimately by the apposition of the two walls a thin lobe is formed, the 

 posterior lobe of the proboscis, which functions as the roof of the passage into the mouth. 

 The point where the two walls touch is the posterior boundary of the glandular part 

 of the ventral wall, which is invariably marked by a line of red pigment, extending 

 transversely from edge to edge. 



It is difficult to explain the functional significance of this red line. It is surrounded 

 by a group of minute nuclei and lightly stained large protoplasmic cells which can be 

 distinguished easily from the glandular cells of the ventral wall. A similar patch of 

 pigment occurs in the oviduct and M'Intosh (1887) suggested that the pigmented 

 oviduct was a phosphorescent organ. Harmer (1905, p. 27) assigns to it a sensory 

 function, though according to him the nerve supply to these regions seemed inadequate 

 for this purpose. In the account of the nervous system it will be seen that there is a 

 distinct nerve originating from the back of the main ganglion which runs to the oviduct, 

 and that the nerve plexus at the base of the glandular region of the ventral wall of the 

 proboscis does not extend beyond the red line. Although no distinct nerve connection 

 with the red line is visible, the nerve plexus may end in the pigmented line by breaking 

 up into very minute branches. The distinct nerve tract in the oviduct suggests that a 

 similar connection exists in the pigmented line of the proboscis, and this definite relation 

 to the nervous system confirms the suggestion of Harmer that the red pigment has a 

 sensory function. This is emphasized by the occurrence of the red line in all known 

 species, where it is prominent from very early stages in the development of the bud. 



The proboscis cavity is traversed by thin strands of protoplasmic fibres and opens 

 externally by two pores, the proboscis pores, which are on either side of the pericardium 

 above the end of the anterior horns of the collar cavities. 



COLLAR CAVITIES 



The two collar cavities together form a ring encircling the pharynx. Dorsally the 

 cavities are large and are separated into the right and left collar cavities by the dorsal 

 coelomic mesentery. They descend downwards and extend into the post-oral lamella, 

 where the two cavities are continuous; there is no ventral coelomic mesentery. The 

 collar cavities open by the right and left collar pores, on either side of the pharynx, at 

 the base of the post-oral lamella, just above the gills. The collar pores are thick-walled 

 ciliated openings which lead by short canals into their respective collar cavities and 

 control the water currents to maintain the turgidity of the collar and arms. 



