STRUCTURE OF GASTREOPODS. 



85 



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 and 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 f 



the Toot. In most of *' 



the aquatic gastero- 



pod mollusks, with a . 



spiral shell, there is l " 



a horny or calcare- P"' 



ous disk or door, o-r j 



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 operculum 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 mollusks 

 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 SNA n.. 



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 ; /. the 

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

 the heart (the pericardium the sack containing 1 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 ? 

 What are eye-stones ? 



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

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

 respiratory tube situated ? 



