C INTRODUCTION. [CH. 



air, by means of lung-like cavities or pouches which they 

 possess. Some kinds of fresh-water snails have a similar 

 apparatus ; but they are also enabled to extract occa- 

 sional supplies of oxygen from the water, and are thus not 

 entirely dependent on their air-pouches. Others of this 

 kind are furnished only with gills, which they use like 

 fishes. In the genus Valvata the gill is external and 

 shaped like a feather ; and the animal has also an au- 

 xiliary branchial organ, which resembles another ten- 

 tacle. The respiratory system of the marine Mollusca, 

 with the exception of a very few littoral species, is bran- 

 chial ; and in some kinds the gills are external. Bivalves 

 have usually two leaf-like gills, which are arranged sym- 

 metrically, one on each side of the body. In the Bra- 

 chiopoda, however, the brachial organs (according to 

 Mr. Hancock) subserve the function of gills, although 

 in one genus (Lingula) the lobes of the mantle may, to 

 a certain extent, be considered specialized breathing- 

 organs. 



There are some peculiarities with respect to habitat that 

 are interesting to geologists. Some kinds of freshwater 

 univalves, both those called Pulmonobranch (i. e. respiring 

 by means of lung-like pouches), and Pectinibranch (i. e. 

 respiring by means of comb-like gills), have the faculty 

 of enduring a partial change or difference in their usual 

 habitat, which would be fatal to other kinds. The 

 Swedish naturalist Nilsson relates that two species of 

 Limncea described by him, as well as Neritina fluviatilis, 

 live in the Baltic, adhering to sea-weeds, and sometimes 

 at a distance from the mouth of any river. With these 

 live certain marine Mollusca, such as the common mussel 

 and cockle, Mya arenaria and Tellina Balthica (or soli- 

 dula\ all of which, however, are of a dwarf size. Lr<m- 

 ncea is Pulmonobranch, and Neritina is Pectinibranch, 



