PHYLUM MOLLUSCA 215 



great number of little teeth arranged in regular rows and an 

 elaborate system of muscles. 



A snail's digestive system is of the tubular type. Food is 

 mixed with the secretion from salivary glands in the mouth, 

 passes down an esophagus into a crop, where it may rest for 

 a time ; then enters the stomach and is mixed with glandular 

 secretions. A curved intestine leads from the stomach to 

 the anus, which, instead of being at the posterior end, is 

 situated in the edge of the mantle, high on the left side of 

 the body. The peculiar position of the anus has been 

 brought about by the twisting of the snail's body in order 

 to become adjusted to the shell. The spiral coil of the 

 shell makes it very economical of space, but has twisted the 

 soft parts of the body to such a degree that some of these on 

 the left side have become rudimentary or absent. 



Physa has a closed circulatory system, with a heart, 

 arteries, veins, and capillaries which distribute the colorless 

 blood to all parts of the body. Blood-vessels are spread out 

 abundantly in the walls of the mantle cavity, and it is there 

 that respiration takes place. Physa is a pulmonate gas- 

 tropod (a snail which breathes air) and the mantle cavity 

 serves as a lung. It comes to the top of the water at inter- 

 vals, protrudes its breathing tube through the surface film 

 (C), takes a deep breath of air, and is then able to remain 

 submerged for some time. On account of the twisting of the 

 body Physa has only one kidney, the right. It consists of 

 a glandular portion, which is close to the pericardial cham- 

 ber, or the space around the heart, and a long tube which 

 opens to the exterior near the anus. 



Self -protection. Physa is protected by its calcareous 

 shell, which furnishes an ever ready retreat in time of 

 danger, and shields it from the cold of winter. The shell 

 also makes it possible to endure drought, and Physa some- 

 times lives for a time in the mud left after a pond has dried 

 up. It is not a protection from all dangers, however, and 

 many fish (perch, pumpkinseed, rock bass, etc.) commonly 

 eat snails whole. 



