104 THE GASTROPODA 
phenomenon may be found both in aquatic species and in forms 
adapted to terrestrial life, the different modifications of the mantle 
being as follows :— 
(1) There may be branchial structures varying in form and 
position, but not homologous to a ctenidium. In the Docoglossa 
these pallial branchiae are situated on the internal face of 
the mantle, as in Patella (Fig. 125, f). In various Gymnosomata 
(Clionopsis, Notobranchaea, ete.) they are situated on the posterior 
surface of the body as in the Pneumodermatidae. In the majority 
of the Nudibranchs they are on the dorsal surface of the body, 
sometimes localised round the anus as in the Doridomorpha 
(Fig. 79), sometimes concealed below a fold of the dorsal integu- 
ment as in Plewrophyllidia and certain porostomatous Doridomorpha, 
Phyllidia, and Corambe (Fig. 164, g). Or all kinds of accessory 
branchial formations may have disappeared, and the function of 
respiration is distributed over the whole free surface of the pallial 
integuments, as may be seen in various Docoglossa such as the 
Lepetidae and Bathysciadium ; in Firoloida among the Heteropoda ; 
in Dermatobranchus, Heterodoris, the Elysiomorpha (with the ex- 
ception of the Hermaeidae), and Phyllirhoé (Fig. 161) among the 
Nudibranchs ; in the Clionidae and Halopsychidae (Fig. 156) among 
the Gymnosomata. 
(2) An adaptation to a terrestrial life and the pulmonary 
respiration resulting therefrom is found in very different groups 
of Gastropods, but the different stages of evolution are best studied 
in the Streptoneura. In this group certain aquatic and littoral 
forms, though they possess ctenidia, have acquired the habit of 
living for a longer or shorter time beyond the reach of the water. 
This is the case with various species of Littorinu (L. rudis, 
L. neritoides, etc.), Cremnoconchus, Neritodryas, several Cerithiidae, 
etc. Consequently certain modifications of the internal surface 
of the mantle are induced, in the interior of the pallial or 
respiratory cavity. The filaments of the ctenidium— bipectinate 
in Neritodryas, but monopectinate in other forms—are often 
reduced in height and are prolonged more or less indefinitely on 
the right side “of the internal pallial surface to form vascular 
arborisations, as may be seen in the semi-aerial species of Lattorma 
(Fig. 85, x) and in Cremnoconchus. Finally, the ctenidium disappears 
altogether, and with it the hypobranchial gland and the efferent 
branchial sinus, and the venous blood of the rectal sinus is con- 
ducted to the afferent cardiac vein (corresponding to the efferent 
branchial vein) by the system of arborisations that extends over 
the whole roof of the pallial chamber. Such is the case in 
Cerithidea obtusa, which retains only the vestiges of the anterior 
extremity of the ctenidium. In many aerial Gastropods the 
ctenidium has totally disappeared and the roof of the pallial cavity 
