78 



PHYTOPHAGA. 



The last order consists almost entirely of terrestrial 

 or fluviatile Mollusea, their organisation being only 

 adapted for respiring free air ; and there are a few 

 fluviatile species found in the second order : the rest 

 are all marine, and therefore excluded from our con- 

 sideration at present. 



ORDER!. PHYTOPHAGA. 



1. 3. Neritina fluviatilis. 

 4. 6. Assiminia Grayana. 

 7. 8. Paludina vivipara. 



9. 1 1 . Bithinia tentaeulata. 

 12.14. Valvata cristata. 

 3. 8. II. 14. The opercula. 



THE gills are in the form of one or more comb-like 

 ridges of plates or filaments on the inner side of the 

 mantle over the back of the neck. The edge of the 

 mantle is entire, and destitute of any syphon. They 

 respire water, or more properly air, through the water, 

 and they are unisexual, and have only two tentacles ; 

 their mouth is usually at the end of a short proboscis, 

 and they live chiefly on vegetable food. 



The shells have an entire roundish or semilunar 



