374 MOLLUSCA. 



The Gastropod ctenidium is (except in Chitonidae) attached by the 

 whole or greater part of its length to the mantle-wall, and carries 

 typically (in the Aspidobranchs and Tectibranchs) two rows of 

 plates arranged perpendicularly to the axis (Fig. 296), but in the 

 more specialized Streptoneura (Fig. 280) one row of plates is absent 

 (it is reduced in the monobranchiate Aspidobranchs). In the former 

 case the gill is said to be" bipectinate, in the latter monopectinate. 



The respiration of air is confined to some Prosobranchs and to the 

 Pulmonata. In this case the mantle-cavity serves as the respiratory 

 cavity, but it differs from the branchial mantle-cavity by containing 

 air, and by possessing instead of a gill a rich network of blood-vessels 

 on the inner surface of its roof. The mantle-cavity communicates 

 by a long slit with the external medium, but in some forms it is very 

 widely open (Tectibranchs), and in others the opening is reduced 

 to a small, round aperture capable of being closed (Pulmonata). 

 Frequently the edge of the mantle is prolonged into a siphon (see 

 above, p. 358). 



The arrangement of the respiratory organs is of importance for 

 the classification of the larger groups. According to their position 

 with regard to the heart two great divisions can, as Milne-Edwards 

 has pointed out, be established: (1) the Opisthobranchiata, in which 

 the auricle and gill are behind the ventricle; (2) the Prosobranchiata, 

 in which the auricle and gill are in front of the ventricle (Fig. 280). 

 So far as this character is concerned, most Pulmonata are Proso- 

 branchiate ; but the Pulmonata in many features of their organiza- 

 tion, particularly in their herrnaphroditism, stand closer to the 

 Op istlwbrancli iata. 



In some aquatic forms the ctenidium is entirely absent, and respiration is 

 carried on either by the mantle and integument generally, without any secondary 

 branchiae (Lepetidae, Dermatobranckus, Heterodoris, many Elysiids, PhyllirJwe, 

 Clionidae, Halopsychidae), or by secondary branchiae ; these may be in the 

 mantle-groove as the branchial plates of Patella, or on the external surface 

 of the mantle, as in Clionopsis, Notobranchaca, and most Nudibranchs. 



Some littoral forms (Littorina, etc.) are able to live for a long time out 

 of water and habitually do so when the tide recedes ; in such cases the internal 

 surface of the mantle may be modified, and in C'erithidea obhisa the ctenidium 

 even completely disappears. In Ampulla/no, one part of the mantle-chamber 

 is modified as a lung, and is partly separated from the other, which contains 

 the ctenidium ; such forms are truly amphibious. Finally, there are the 

 so-called laud Operculates ; these are streptoneurous forms in which the ctenidium 

 has disappeared and the mantle-cavity is completely transformed into a lung. 

 Of such forms we have the rhipidoglossate family the Helicinidae, and three 

 families of the Taenioglossa, viz., the Cyclophoridae, Cyclostomatidae, Aciculidae. 



In the Pulinonuta proper the ctenidium is entirely absent and the opening 



