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 pal Hal branchiae are situated on the internal face of 

 the mantle, as in Patella (Fig. 125, /). In various Gymnosomata 

 (Clionopsis, Notobranchaea, etc.) they are situated on the posterior 

 surface tf 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 Pleurophyllidia 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 Phyllirhoe (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 Littorina (L. 'nidis, 

 L. neritoides, etc.), Cremnoconchw, 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 Littorina 

 (Fig. 85, x) and in Crcmnoconchus. 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 



