VII MOLLUSCA~THE ALIMENTARY CANAL 177 



with the pancreas, and perhaps combines the functions of the different 

 specialised digestive glands of Vertebrates. 



There is a radical difference between Lamellibranchs and other 

 Molluscs,! in the fact that in the latter the anterior portion of the fore- 

 gut which follows the buccal cavity is developed as a muscular pharynx 

 (oesophageal bulb, buccal mass), and carries at its base on a movable 

 lingual cushion a file-like organ, the radula, which is beset with 

 numerous hard teeth composed of conchyolin or chitin. The radula 

 serves chiefly for mastication, but is sometimes used in seizing, holding, 

 and swallowing prey. 



None of the Lamdlihranchs have a pharynx provided with a radula, 

 they are therefore called Aglossa as opposed to all other Molluscs, which 

 are Glossoplwra. 



Hard jaws, composed of conchyolin, are almost always found in 

 varying number and arrangement in the buccal cavity of the Glosso- 

 phora, but are wanting in all Lamellibranchs. 



One or two pairs of glands open into the pharynx in the Glossoplwra; 

 these are usually called salivary or buccal glands, although they very 

 slightly if at all correspond physiologically with the glands so named 

 in the Vertebrata. Glands may also open into the buccal cavity. The 

 Lamellibranchs have no salivary glands. 



The absence of the pharynx, tongue, jaws, and salivary glands in 

 the Lamellibranchia is accounted for by their manner of life. They do 

 not have to seek their food. Some of them are attached and others 

 feed in the same way as attached animals on small particles suspended 

 in the respiratory water (animalculse, microscopic algee, and particles of 

 detritus) which are brought to the mouth by means of ciliary move- 

 ment. These fine particles require no mastication before being 

 swallowed. 



This method of feeding also affects the outer organisation of the 

 Lamellibranchia, which have lost the cephalic portion of the body with 

 the tentacles and eyes : Aglossa = Aeephala = Lipoeephala, and 

 Glossophora = Cephalophora. 



In some Gastropoda (Murex, Purpura) and in Dentalium there is in 

 connection with the last part of the hind-gut an anal gland, and in 

 the Cephalopoda (excepting Nautilus) a gland known as the ink-bag. 



The alimentary canal of the Molhisca runs through the primary 

 and often also through the secondary body cavity, attached in various 

 ways by fibres or bands of connective tissue. Its walls consist of an 

 inner epithelium usually to a great extent ciliated, an outer muscular 

 layer in which longitudinal and circular fibres occur, not always in 

 regular layers, and, where it passes through the primary body cavity, 

 an outer envelope of connective tissue. 



The pharynx and perhaps sometimes part of the oesophagus, and a 

 part, in all cases very short, of the hind-gut, arise ontogenetically out of 

 the ectodermal stomodaeum and proctodaeum. But the exact limits 



1 For the rare exceptions to this rule, see p. 183. 

 VOL. II N 



