CHAPTER XXXVII 



ANIMAL RESPIRATION— BACKBONELESS ANIMALS 

 WHICH BREATHE IN AIR 



Having considered the backboned air-breathers, we now pass 

 on to Molluscs, Arthropods, Worms, &c. which live on land or 

 in fresh water, and use ordinary air in breathing, as contrasted 

 with members of the same great groups that live in water, salt or 

 fresh, and respire the air which is dissolved in this. 



AIR-BREATHING MOLLUSCS (Mollusca) 



The only subdivision of the Molluscs which contains air- 

 breathing members is the class (Gastropoda) that includes Snails 

 and Slugs of all kinds. The great majority of these are marine, 

 but the order of Lung-Snails (and Slugs) has been established for 

 the reception of most of the species which live on land or, it may 

 be, in fresh water, and are collectively termed Pulmonates (L. 

 pidmo, a lung), because they breathe air by means of an organ 

 which may be called a " lung ", though it is quite different in 

 origin and nature from the lungr of a backboned animal. There 

 are also certain land-snails {Cyclophorus, Cyclostoina, &c.) which 

 are not Pulmonates (though they breathe in a similar way), but 

 belong to the Fore-gilled Gastropods (Prosobranchs). 



We have seen (see p. 393) that in the marine Prosobranchs 

 there is a cavity opening by a wide slit above the neck, and con- 

 taining either two (Ormer and Keyhole Limpet) or more usually 

 but one (Whelk and Periwinkle) plume-like gill. This is the gill- 

 cavity, and its roof is the mantle, which may be regarded as a sort 

 of flap that has grown out from the wall of the body. In an 

 ordinary Limpet [Patel/a) the gill-cavity has lost both its gills, and 

 is probably able, by means of its thin roof (which is richly pro- 

 vided with blood-vessels), to breathe the damp air which surrounds 



