26 LAND AND FRESH-WATER SHELLS. 



buccal mass of some snails. The epithelium in the buccai mass 

 is cylindrical, and in some places ciliated. 



From the buccal mass a narrow gullet (oesophagus) passes into 

 a distended portion of the enteric tract, the crop, on the sides 

 of which are placed the salivary glands. A microscopical examina- 

 tion of a section of the oesophagus presents six layers from without 

 inwards, (i) a peritoneal investing coat; (2) a layer of longitu- 

 dinally arranged muscle-cells ; (3) a layer of circularly arranged 

 muscle-cells ; (4) a lacunar mucosa layer ; (5) a layer of epithe- 

 lium ; (6) a lining cuticle. * The crop is spindle-shaped (fusiform), 

 with its lining membrane thrown into numerous longitudinal 

 folds ; it is generally conspicuous on account of the yellow 

 character of its contents. The salivary glands are compound 

 glands made up of unicellular glands. They are generally two in 

 number, but H. pomatia has an additional one imbedded in its 

 buccal mass. They secrete free sulphuric and free hydrochloric 

 acids, which act upon the starchy matters of their food-stuffs in 

 the same way as the ptyalin of the human saliva. They convert 

 the starch into sugar. This secretion is carried from the glands 

 by two ducts, which open into the buccal mass just above the 

 odontophore. The glands will be seen to be lobulated, and con- 

 fluent with one another on the posterior and dorsal surface of the 

 crop. 



Passing from the crop the enteric canal narrows, but soon 

 expands again to form the stomach. This is a simple sac, incom- 

 pletely subdivided into two by a longitudinal septum, and into 

 which the ducts of the " Mitteldarmdrilse " open. The stomach 

 differs in contour in different species. In some marine forms, as 

 Aplysia and Bullcea aperta, horny processes and plates are deve- 

 loped, with which to triturate the food, a condition which strongly 

 reminds one of the " gastric mill " of a cray-fish. In the former 

 of these, Aplysia, the wall of the stomach is thickened, and 

 beset with horny spines like canine teeth, and with rhomboidal- 

 shaped plates like molar teeth. The food, consisting of seaweeds, 

 after having been coarsely masticated in the buccal cavity and 

 crop, is pierced by these spines and afterwards pounded into a 

 pulpy mass by the plates. In the second of these, Bullcea aperta, 

 there are three large plates, which are convex on the outside and 



