STRUCTURE OF GAfeTREOFODS. 



ap p 



3. The mass of the viscera occupies the upper part of the 

 cone formed by these shells, and always remains enclosed in it , 

 but the head arid foot project externally when the animal extends 

 itself to walk, and return again into the last turn of the spire, 

 when it contracts : and the size of this latter part of the shell and 

 the form of its open- 

 ing are always in 



relative proportion to / 

 the foot. In most of 

 the aquatic gastero- 

 pod rnollusks, with a . 

 spiral shell, there is 

 a horny or calcare- P"' 

 ous disk or door, 

 called operculum 

 (fg. 32, o), which 

 is fixed to the poste- 

 rior part of the foot, 

 and which closes the 

 entrance of the shell, 

 when the animal 

 retires into it. 

 The operculvm of certain small shells constitutes what is vulgar 

 ly termed an eye-stone. 



4. The organs of respiration are formed sometimes for breath- 

 ing the air, and sometimes for aquatic respiration ; but in rnollusks 

 with a spiral shell, they are always lodged in the last turn of the 

 spire, and receive the ambient element beneath its edge, either by 

 a hole pierced through the mantle, or between the body and the 

 free edge of this cutaneous fold, which is also often prolonged 

 into a canal, by means of which the animal can obtain the fluid 

 (whether air or water) necessary for its respiration, without 



---- ! 



e d pi 



21. ANATOMY OF A SNAIL. 



Explanation of Fig. 21. Anatomy of a snail ; pi. the foot; t. the tenta- 

 cles partly contracted ; d. a sort of diaphragm which separates the respira- 

 tory cavity from the other viscera ; e. a portion of the stomach ; -f. the 

 liver ; o. the ovary ; i. the intestines ; r. the rectum ; a. the anus ; e. 

 the heart (the pericardium the sack containing the heart being open ; 

 ap. the pulmonary artery ramifying on the parietes of the pulmonary cavity 

 (p} ; v. the gland which secretes the viscosity ; cv. its excretory canal, 

 going to open near the anus. 



3. In what part of spiral shells are the viscera of the animal contained ? 

 What part of the shell contains the head and foot? What is the operculum f 

 What are eye-stones ? 



4. In what part of the spiral shells are the organs of respiration placed 7 

 By what means is the air or water admitted to these organs ? Where is the 

 respiratory tube situated ? 



