Boycott : the habitats of freshwater mollusca. 249 



contains plenty of calcium for large colonies of heavy Anodonta shells, 

 though it may well be that a superabundance is advantageous. It is 

 possible, though unlikely, that there is a directly limiting tactor cap- 

 able of crude chemical expression, e.g., copper, manganese, 'i'lie 

 supply of oxygen is hardly of direct nnportance for pulmonates ; for 

 the bivalves, operculates and such species as Aiicylus Jiuvtatilis, it 

 must be a paramount consideration. Oxygenation depends partly on 

 the movements of the water, partly on the respiratory processes of 

 plants, and, as concerns any one group of animals, on the presence 

 of organic matter, live or dead, which competes for what oxygen is 

 available. We are badly in need of definite analytical data on the 

 gaseous content^ of closed ponds. 



We may here note that an analysis of our local records, taking 

 closed ponds as forming a tolerably homogeneous group, indicates 

 the close association between snails and plants. On the average, 

 ponds with no snails show 17 sorts of the larger water plants, and 

 both numbers increase together till the figure for ponds with five or 

 more sorts of snails reaches 5 "4 sorts ot plants. This is a sadly 

 meagre quantitative measure, but there is enough evidence to show 

 that the correlation will prove a fruitful study in many ways. Thus 

 the, fauna of drying ponds may be determined by the consequent 

 destruction of plants as much as by the capacity of snails themselves 

 to withstand dessication. 



(5). Biological factors crudely fall into the two divisions of food 

 and enemies. We know practically nothing of either. There are 

 grounds for thinking that water snails feed on the larger plants as 

 little as do land snails, and that algae and other small things form 

 their main diet, though their relation to the higher plants is likely to 

 be altered when these are partially decayed. Of any demands made 

 by particular sorts of snails for particular kinds of food we know 

 nothing. One of the most deplorable lacunae in our knowledge of 

 how the world is made is the causation of the natural mortality of 

 live things ; we are indeed mostly ignorant of whether and when they 

 die or live, except in the roughest qualitative fashion. It would be 

 impossible to overestimate the value of precise quantitative reasons 

 for the fact that our ponds are not solid with snails. 



Summarising the discussion of the inferiority of closed ponds, it 

 seems that two pretty definite factors can be disentangled — the sup- 

 ply of oxygen and the opacity of the water. The former may be 

 presumed to account in some measure for the habitats of Ancylus 

 fiuviatilis, Fhysa fontinalis, Planorbis coniortus, perhaps Fisidium 

 amnicu?n, and experiment shows that these species die away in jam- 

 pot aquaria very quickly in comparison with e.g., Flanorbis spirorbis 



