THE MOLLUSCA 107 



These curious Naked-gilled Molluscs are very numerous ; over 

 a hundred species exist on the British coasts alone, but they 

 are mostly very minute, and often so transparent that they are 

 not readily seen. As a rule, these creatures are carnivorous, 

 feeding chiefly on other molluscs or sea-anemones, while Doris 

 has a strange liking for sponges. 



In the Air-breathing Molluscs (Pulmonata) the gill is replaced 

 by a lung or, more correctly, a lung-cavity, for the lung of the 

 mollusc is not identical with the lung of a vertebrate animal, but 

 consists of a pouch or cavity, lined with a network of blood-vessels, 

 termed the pulmonary sac. Land and fresh-water Molluscs were, 

 in all probability, originally marine forms, which, by a gradual, 

 natural process, have passed through varying stages from water- 

 breathers to air-breathers. Nature, of course, does not stand still, 

 and this process of evolution is still going on to-day ; so we find 

 certain forms of Molluscs in an intermediate or transitory state 

 possessing branchial gills, in a more or less evanescent condition, 

 in addition to the pulmonary sac. One group of air-breathers 

 live principally in salt or brackish water ; and in the tropics 

 huge species of these Molluscs are found living among the mangrove 

 roots at the mouths of rivers, while others frequent damp woods 

 close by the sea. 



Fresh-water Pulmonata, although living in water, are not 

 furnished with gills, and as a rule rise periodically to the surface 

 to take in a fresh supply of air. Nevertheless, many Water Snails 

 seem perfectly able to sustain life under water without rising for 

 quite long periods, and it is probable that in such cases the mollusc 

 respires through the skin, as do some of the Nudibranchs; but 

 opinion is divided upon this subject. 



Land Snails have a well-developed shell, into which they are 

 able entirely to withdraw. In the Slugs the shell is usually rudi- 

 mentary or altogether absent ; it may be external or internal, 

 in some species taking the form of a small, shield-like plate covering 

 the breathing organs. 



All terrestrial Molluscs hibernate at some period of the year. 

 In hot countries they retire in the dry season, coming out again 

 with the first rain. In cold countries they hibernate during the 

 winter months, seeking before their long sleep some secluded spot 

 under heaps of leaves or moss, beneath stones, in cracks in old 



