MOLLUSCA~THE 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 pouch, 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, c-c, Cephalic intesuineiit ; c, edge of the aperture of tlie 

 proboscidal slieath ; c-d, Inuuovable wall of the proboscidal sheath ; d-c, 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 proboscis ; g, pharynx ; A, oesophagus ; i, retractor muscle ; /,-, salivary 

 glands ; /, 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 Tcrehridce). 



In the Hetcropoda, 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. 



