386 



MOLLUSCA. 



B 



some Gerithiidae, the Melaniidae, Gremnoconchus, Canidia. The 

 Pulmonata Stylommatoplwra and certain Streptoneufa (Helicinidae, 

 Cydoplioridae, Cydostomatidae, Aciculidae) are terrestrial. Many 

 Littorina, Ceritlrium, Melania, etc., live in brackish water. Many 

 branchiate Gastropods are able to live for some time out of water 



in dry places ; in such circumstances 

 they are withdrawn into their shells, 

 the opening of which is closed by 

 the operculum. Almost all move by 

 creeping ; some, however, as Strom- 

 Jms, jump ; others, as Oliva and 

 Aiii'lUaria swim excellently by the 

 aid of the lobes of their foot. Some 

 marine forms, e.g., Magilus, Vermetus, 

 etc., are fixed by their shells ; a few 

 only are parasitic, as Stilifer on Sea- 

 urchins and Starfishes, Entoconcha in 

 Synapta. The method of nutrition 

 differs as much as does the habitat. 

 Many, especially the siphonostomate 

 forms, are voracious predatory animals, 

 and prey on living animals ; some, as 

 Murex and Natica, with this object 

 bore into the shells of Molluscs ; 

 several (Strombus, Buccinum) prefer 

 dead animals. An equally large num- 

 ber, viz. almost all Pulnionates and 

 holostomatous branchiate Gastropods, 

 feed on plants. More than 17,000 

 species are known : they are found 

 on all parts of the earth ; in the deep 

 sea to a depth of 2650 fathoms, and 

 from the Himalayas above the snow 

 line. Geologically they are found 

 at the very commencement of the 

 palaeozoic epoch in the Cambrian, and the genus Heli.c (Zonites 

 Montf.) is known in the Carboniferous. 



FIG. 304. Shells of A Pleiirotomaria, 

 B Polytremaria, C and E Emarginula, 

 D Haliotis, F Flssurella, G and H 

 stages in the growth of the shell of 

 Fissurella. 



