THE GASTROPODA 



103 



forms in which the mantle has disappeared as a shell -forming 

 organ, from the dorsal envelope of the body, which in this case 

 serves as an accessory respiratory organ. This phenomenon is to 

 be seen in the Acmaeidae (Fig. 82, pa.v), the Heteropoda, the Pleuro- 

 branchidae, and the Pneumo- 

 dermatidae, these last-named 

 families being naked. In the 

 Pleurobranchidae, the Hetero- 

 poda, and certain Acmaeidae 

 the mantle is no longer fur- 

 nished with accessory respira- 

 tory structures, but in other 

 Acmaeidae, such as Scurria, 

 etc., and certain Pneumoder- 

 matidae (Fig. 84, VI, VII), a 



FIG. 84. 



FIG. 83. 



Tritonia lineata, dorsal view. 

 I, rhinophore or posterior 

 tentacle ; II, dorsal appendage 

 (pallial gill); III, right eye", 

 IV, frontal veil; o, genital 

 (hermaphroditic) orifice. (After 

 Hancock.) 



VII 



right - side view, 

 with the head above. 1, the expanded 

 proboscis ; II, anterior tentacle ; III, 

 posterior tentacle ; IV, genital (herma- 

 phroditic) opening : V, right tin ; VI, 

 ctenidium ; VII, posterior pallial gill ; 

 VIII, posterior lobe of the foot; IX, 

 reno-anal cloaca ; X, lateral margin of 

 the foot ; XI, penial orifice ; XII, 

 sucker-bearing appendage ; XIII, ven- 

 tral median papilla of the proboscis ; 

 XIV, seat of the mandibles ; XV, ex- 

 panded right hook-sack. 



ctenidium, or branchia properly so called, coexists with secondary 

 respiratory organs or pallial branchiae : these lie below the mantle 

 edge in Scurria and on the free surface of the posterior part of the 

 body in the Pneumodermatidae. If the ctenidium is atrophied and 

 disappears altogether, the mantle itself resumes the respiratory- 

 function which was previously localised in the ctenidium. This 



