22 LAND AND FRESH-WATER SHELLS. 



Blundstone has found a substance named glycogen in JItHx, and 

 also in the fresh-water mussel (Anodo/i), an observation to which 

 we shall have to refer when treating about the physiology of the 

 snail. The matrix contains stellate-shaped cells, and in Amalia 

 carinata and Liniax flavns Leydig has observed coloured cells, 

 termed chromoblasts^ which change their contour under the influ- 

 ence of suitable re-agents. Mucous glands, unicellular in cha- 

 racter, are scattered all over the body of all Pulmonates, together 

 with goblet-shaped cells and sensory cells. The mucous glands 

 are in great quantity on the collar of the mantle ; they secrete 

 mucus, and it is this mucus which forms the threads by which the 

 slugs and the LimncEa, among land and fresh- water molluscs, and 

 the Litiopa and Rissoa parva^ among marine forms, suspend 

 themselves. 



Running along the foot in all Pulmonates is a ccecal diverti- 

 culum of the integument termed the supra-pedal gland. In 

 Cy do stoma elegans there are two. In Avion and Geonialacus a 

 large slime gland is present at the posterior extremity of the tail ; 

 in Avialia and Liniax this structural feature is absent. The 

 epithelium of the supra-pedal gland is ciliated, and has been said 

 to contain sense cells. 



Pin the animal down under water, and reflect the mantle and 

 the integument of the prostoma. Dissect the following systems 

 out in order : — 



The Digestive System. — The digestive system commences at 

 the mouth, already noticed, enlarges to form a buccal mass, then 

 passes back by an oesophagus to form a crop and stomach, and 

 from thence proceeds through the substance of the digestive 

 gland, or " Mitteldarmdriise," as Frenzel terms it, as the intestine 

 to end at the anus. The buccal mass or pharynx is a stout 

 muscular cavity, situated in the head segment, from tiie lower and 

 posterior surface of which a pale diverticulum depends, — the sac 

 qf the radula. This buccal mass contains the masticatory organs. 

 These consist of a horny beak lying in the circular lip, and of an 

 odontophore lying below a yellow ribbon containing teeth, known 

 as the radula. 



The beak plays the function of a mandible in grasping the vege- 

 table or animal substance on which the animal feeds, while the 



