MOLLUSCA. 127 



wall of the alimentary canal, outside the epithelium, contains 

 muscular layers, an internal longitudinal and an external circular. 

 The salivary glands are aggregates of unicellular glands, each of 

 which is sharply marked off from its neighbours, and has its 

 own minute duct. The digestive gland is composed of branched 

 tubules, ending blindly, and lined by glandular epithelium, in 

 which three chief types of cell can be distinguished, (1) granular 

 cells (liver-cells), containing yellowish granules, (2) pear-shaped 

 ferment cells, (3) large calcareous cells. 



The snail chiefly feeds upon the fresh leaves, steins, &c., of 

 plants, from which it rasps off small fragments by means of its 

 radula, in the following way : The buccal mass is pulled forwards 

 by means of its protractor muscles, when the front of the radula 

 on its cushion projects a little from the mouth. Appropriate 

 contractions of the small muscles within the cushion move the 

 sub-radular membrane, and with it the radula, backwards and 

 forwards, the jaw meanwhile holding the food firmly, and serving 

 as a relatively fixed part against which the radula works. The 

 particles of food scraped off pass back into the mouth-cavity, 

 partly by the agency of the flexible lips, and partly as a result 

 of the backward movement of the odontophore, which acts to 

 some extent like a suction-pump. The salivary secretion has 

 been stated to contain a ferment which converts starch into 

 sugar, and the acid secretion of the digestive gland is known to 

 bring about fermentative changes of this kind and others involving 

 the conversion of proteids into peptone. The food gets mixed 

 up and gradually passed backwards by the contractions of the 

 muscular walls of the gut, the length of which gives a consider- 

 able absorbing surface, augmented by the longitudinal fold in the 

 intestine. 



The digestive gland has other functions besides that of aiding 

 digestion. The granules in its granular cells are most likely of 

 excretory nature, and the material secreted by its calcareous cells 

 is used in the construction of the epiphragm. 



4. Circulatory Organs. As in the mussel, a blood system alone 

 is present, and the reduced ccelom is represented by the pericardial 

 cavity. Heart, arteries, and venous system can be distinguished. 

 The blood is of a bluish tinge owing to the presence of haemo- 

 cyanin, and consists of plasma and amoeboid colourless corpuscles. 



The muscular heart is situated in the posterior part of the 



