of Paludestrina ulvre and ventrosa. 527 



C. Fourth secondary ditch from west (closed) (four stations : 

 May-September) : 



Upper end — 



Paludestrina ventrosa (swarming). 

 Ruppia maritima (May-July). 

 Euteromorpha intestinalis") , . , 



Vauchei ia sp. j ^ ° '' 



Salinity : 2'78 / o -2'9 % NaCl. 



Lower end — 



Paludestrina ulvce (swarming). 



ventrosa (moderately numerous). 



Viva lactuca. 



Salinity: 2'78 °/ -2-95 % NaCl. 



From the above lists it will be seen that Paludestrina ulvce 

 appears to be limited to water in which Ulva occurs. 

 Stragglers may be found upon other plants {Enter omorpha 

 intestinalis and Schlerochroa maritima) in this area, but it 

 attains its maximum upon Ulva. P. ventrosa, on the other 

 hand, appears to be less restricted in its distribution, as it 

 occurs in quite appreciable numbers in all three ditches and 

 upon a variety of plants. It therefore may be reckoned as 

 more plastic and adaptable than P. ulvce, though it un- 

 doubtedly thrives in permanently brackish water — an assump- 

 tion borne out by its absence from typically estuarine faunas. 



Attempts were made with artificial sea- water to discover 

 the lowest degree of salinity that P. ulvce would tolerate. 

 For reasons given below it was impossible to arrive at any 

 precise figures, but this much was satisfactorily ascertained — 

 that in water under '5 °/ NaCl the animal contracted imme- 

 diately upon immersion and never emerged from its shell as 

 long as it remained in that water, while in water of 1*0 °/ 

 NaCl it showed no obvious discomfort upon immersion and, 

 on the whole, behaved very much as it did in higher salinities. 

 Placed in water from C (upper end), from which it is 

 normally absent, it behaved in its ordinary fashion. 



In the course of these attempts a curious confirmation was 

 obtained of the view that P. ventrosa is more adaptable than 

 P. ulvce. The behaviour of the two animals in captivity is 

 very different. P. ventrosa proved itself a very satisfactory 

 subject, as it always remained in the water in which it was 

 placed. P. ulvce, on the other hand, was invariably intrac- 

 tible. It usually crawled out of the water, and, if possible, 



3G* 



