52 THE LIFE OF THE MOLLUSCA 



laminarian zones (i.e., between tide-marks ; and so 

 far as the seaweeds grow, or to about fifteen fathoms) ; 

 a smaller number inhabit the deeper nullipore or 

 coralline zone ; whilst a few stragglers are met with 

 at great depths. In all cases the nature of the sea 

 bottom governs their individual distribution. Certain 

 forms frequent the rocks, others sandy or muddy sea- 

 floors. Some, on the other hand, spend their lives 

 in the surface waters of the open sea. 



The brackish waters of estuaries and lagoons are 

 tenanted by a few kinds, including the strange pul- 

 monate Amphibola (Plate XXVI., Fig. 23) ; while 

 rivers, streams, and lakes are the dwelling-places of 

 a considerable number. On the land, every spot 

 capable of supporting life yields its quota of Mollusca, 

 and the total number of known terrestrial species is 

 consequently very great, and yearly being added to. 



As will presently be mentioned, some forms will 

 normally trespass out of their regular habitat, but cer- 

 tain more exceptional cases of interchange between 

 marine and freshwater haunts may be appropriately 

 alluded to here. Freshwater Snails sometimes 

 become accustomed to salt-water conditions ; thus, 

 at Bornholm, in the Baltic, specimens of Limncea 

 and Theodoxis have been found living in company 

 with marine molluscs in water containing as much 

 as 1 to i'5 per cent, of salt. In Southern Algeria 

 Melania and Melanopsis inhabit waters surcharged 

 with salt, where the marine Cockles failed to survive. 



