BEANCHI/E OF MOLLUSCA. 339 



lose their apparently specific structure, and get to be more and more 

 like the surrounding integument, of which they form processes varying 

 greatly in character. Their relations to the circulatory apparatus are 

 of great significance as bearing on this view, for they so far agree 

 completely in character with the true gills. Lastly, when most 

 differentiated, the gills are seen to bo distributed over the whole 

 of the dorsal region of the body, where they form one or more rows 

 of papilla3, or villous processes on either side ; these again may bo 

 branched (^Eolidise). The loss of the shell is the cause of the 

 wider distribution of the gills, just as, on the other hand, the forma- 

 tion and development of this organ of defence was the cause of the 

 gills being more limited in extent. 



These gills are atrophied in many Opisthobranchiata, when the 

 whole of the integument takes on the respiratory function (Phyllirhoe, 

 Elysia, Pontolimax) . 



§ 262. 



Another arrangement of the respiratory apparatus, which is a 

 modification of the one first described, is due to the development of 

 the respiratory canal-system in the walls of the mantle -cavity. In 

 many of the branchiate Gastropoda this network of canals extends 

 beyond the gills into the "neighbouring parts of the branchial cavity, 

 which are thereby enabled to take part in the respiratory function. 

 In this way the mantle-cavity is adapted to taking in air, and 

 becomes a lung. An organ of this kind — which is not at all adapted 

 for those Molluscs that are so organised as to be fit for an aquatic life 

 — is found in various forms, which belong to very different divisions, 

 and it is to be regarded as due to a change in their mode of life. A 

 lung is present, in addition to a gill, in Ampullaria ; in this animal 

 it forms a sac, which is placed by the side of the gill, and is pro- 

 vided with a contractile orifice. In the terrestrial genus, Cyclostoma, 

 the gill has disappeared altogether. 



There is a lung in Onchidium, but it is also a renal organ. A 

 similar cavity is found close to, and has the same orifice as, the 

 renal organ in the Helicinae and Limacinaa ; this functions as a lung. 

 In the LymnaBidoe and Planorbidre, however, the mantle-cavity itself 

 is adapted to the reception of air. But in these forms the abran- 

 chiate mantle-cavity also serves as a water-breathing organ, for 

 many Lymnasidas are known to live always in deep water. 



§ 263. 



In the Gymnosomatous Pteropoda either the whole of the integu- 

 ment (Clio) serves as an organ of respiration, or processes are 

 developed from its surface which function as gills (Pneumodermon) . 

 In the thecosomatous forms only do we meet with plaited folds 

 (Hyalea), which are placed in the mantle-cavity (Fig. 171, A br), and 

 so far resemble the arrangements which obtain in the rest of the 

 Mollusca ; their position is the same as that of the gills in the 



