30 



COMPARATIVE ANATOMY 



CHAP. 



C. Gastropoda (Cephalophora). 



Although there can be no doubt as to the relationship to one 

 another of the Mollusca grouped together 'in this class, it is almost 

 impossible to give a general scheme of the outer form of the whole 

 class. The greatest variation occurs, the body being sometimes out- 

 wardly bilaterally symmetrical, sometimes in a high degree asym- 

 metrical. Further, forms such as Fissurdla, Oliva, Turritella, Cleodora. 

 Pterotrachea, Phyllirhoe, Limax, Pleurobranchus, Thetys, differ so greatly 

 in outward appearance that, at the first glance, it is almost impossible 

 to believe that they are related. A shell may be present, and may show 

 the most marvellous variation in form ; or it may be rudimentary or 

 even (in adult forms) altogether wanting. The foot also may assume 

 the most varied forms, or may be entirely wanting. The same may 

 be said of the mantle fold, the gills, etc. 



Setting aside those forms which are quite one-sidedly differenti- 

 ated, it may be said in general (1) that, in the Gastropods, the 

 protective shell consists of one piece, and follows in a remarkable way 

 the forms assumed by the body ; (2) that the dorsal portion of the 

 body, which contains the viscera, becomes constricted almost hernia- 

 like from the head and foot, making a sac-like protuberance (visceral 

 dome) ; (3) that, for the diminution of its surface, this dome or hump 

 becomes coiled spirally, the shell repeating its shape ; (4) that the head 

 and foot, which project through the aperture of this shell for purposes 



of locomotion, can be withdrawn 

 into it. The large, long foot 

 generally has a flat sole for creep- 

 ing. The head is distinct, and 

 provided with tentacles and eyes. 

 At some part of the body, the in- 

 tegument of the visceral dome 

 forms a mantle fold which hangs 

 downwards, covering and protect- 

 ing the respiratory organs. The 

 outer surface of this mantle takes 

 part with the rest of the integu- 

 ment of the visceral dome in the 

 formation of the shell. The follow- 

 ing are more special descriptions of 

 the outer organisation of the chief 

 Gastropodan groups. 



FIG. 40. Diagram of the Organisation of a 

 Zeugobranchiate Diotocardian. a, Anus ; ve, 

 ventricle ; ula, right auricle ; nrct, left ctenidium ; 

 uros, left osphradiuin. 



1. 



Prosobranehia. 



The large visceral dome is 

 coiled spirally, generally to the 

 right (dextrally), the shell naturally assuming the same form. The well- 



