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 Scwiria, 
ete., and certain Pneumoder- 
matidae (Fig. 84, VI, VII), a 
Fia. 84. 
Pneunonoderma, right-side view, 
with the head above. I, the expanded 
proboscis; II, anterior tentacle; III, 
posterior teutacle; IV, genital (herma- 
phroditic) opening; V, right fin; VI, 
Fig. 83. etenidium ; VII, posterior pallial gill; 
Tritonia lineata, dorsal view. VIII, posterior lobe of the foot; IX, 
I, rhinophore or _ posterior reno-anal cloaca; X, lateral margin of 
tentacle; II, dorsal appendage the foot; XI, penial orifice; XII, 
(pallial gill); III, right eye; sucker-bearing appendage; XIII, ven- 
IV, frontal veil; o, genital tral median papilla of the proboscis ; 
(hermaphroditic) orifice. (After XIV, seat of the mandibles; XV, ex- 
Hancock.) 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 
