VII 



MOLLUSCATHE ALIMENTARY CANAL 



179 



whole length of the freely-projecting egg-shaped snout, which carries leaf-like labial 

 appendages. On each side of the buccal cavity there is a ponch, the so-called cheek 

 pouch, which is lined with glandular epithelium and opens into the cavity anteriorly. 



FIG. 152.- Diagram of the proboscidal apparatus of the Prosobranchia. A, proboscis 

 retracted. B, The same protruded, u-c, Cephalic integument ; c, edge of the aperture of tin' 

 proboscidal sheath ; c-d, immovable wall of the proboscidal sheath ; d-e, movable (evaginable and 

 invaginable) wall of the same ; e-f, immovable wall of the proboscis ;/, edge of the oral aperture, at 

 the anterior end of the'proboseis ; g, pharynx ; h, oesophagus ; i, retractor muscle ; k, salivary 

 glands ; I, cephalic cavity. 



An exact comparative investigation of the mechanism of the proboscidal 

 apparatus, the contractile snout, etc. of the Prosobranchia is still a desideratum. 



There are other forms of proboscis, differing greatly from the one just described 

 (e.g. that in the Tcrebridce). 



In the Heteropoda, the head forms a long snout which is often described as a 

 proboscis. The name is inappropriate, as this snout is not retractile and the mouth 

 is always found at its anterior end. 



