GENERAL ZOOLOGY 



Retractor muscles 

 of radula 



Esophagus 



Radula 



Radula 

 protractors 



^Odontophore 

 protractors 





1/ \ 



'f*"-' #i^' >' ^' ^- 



!^' &^^ &" C" i^ i 



Fig. 13.12. The e;astropod radula. .4, radula of the snail Bmycon, showing relationships of 

 the muscles which operate it; diagrammatic. B, photomicrograph of the surface of the radula of 

 a fresh-water snail. {A, redrawn from M. Pierce in F. A. Brown, Jr., el al.. Selected Inverte- 

 brate Types, copyright 1950 bv John Wiley and Sons, Inc., printed by permission; B, photograph 

 courtesy General Biological Supply House, Inc.) 



snails, two fundamental changes have occurred in the course of their evolu- 

 tion. One of these developments involved a dorsal elongation and flexure of 

 the visceral mass, eventually accompanied by spiral coiling. This has resulted 

 in the production of the characteristically coiled, elongated visceral hump of 

 most snails, containing part of the intestine, the digestive diverticulum, and the 

 gonad. The other process consisted of a rotation, or torsion, about the 

 dorsoventral axis of the body. This brought the originally posterior mantle 

 cavity, containing the gills, the anus, and the openings of the excretory and 

 reproductive ducts, around to lie anteriorly above the head, in some cases, 

 or to an anterolateral position. The evidence for the occurrence of this 

 torsion is found in the consequent adjustments and alterations of internal 

 anatomy, particularly as they have involved the nervous system. Such 

 evidence also indicates that a certain amount of "detorsion," or reversal of 

 rotation, has occurred in many forms whose ancestors had undergone com- 

 plete torsion. 



Some gastropods have shells which are not coiled; this probably represents 

 a secondary condition, because the internal organs show evidences of coiling, 

 and related species have somewhat coiled shells. An interesting type showing 

 greater coiling in the visceral mass than in the shell is represented by 

 species of the genus Crepidula, the "boat shells." These are sedentary snails 

 which feed by the use of cilia and sheets of mucus. This method of feeding 

 is unusual among gastropods, most of which use the tongue-like radula (Fig. 

 13.12). This organ consists of a ribbon of tissue bearing rows of horny 

 teeth; by the operation of a complicated arrangement of specific muscles, this 



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