52 THE LIFE OF THE MOLLUSCA 
laminarian zones (.¢., 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 Limnaea 
and Theodoxis have been found living in company 
with marine molluscs in water containing as much 
as I to 1°5 per cent. of salt. In Southern Algeria 
Melania and Melanopsis inhabit waters surcharged 
with salt, where the marine Cockles failed to survive. 
