Retrospective Criticism. 399 



Magazine (p.l21. et seq.), do not conclusively show that that animal will not 

 live in fresh water ; for there are many marine animals that die on being 

 immersed in fresh water, which may be made to live in it, if the sea water, 

 which is their natural element, be gradually diluted with fresh water, until 

 the whole has, in fact, become fresh. Dr. Drummond will perform a useful 

 service to this branch of zoological enquiry, by subjecting the NereiV caerulea 

 to experiments of this description. 



With respect to the cause of the deleterious effect of freshwater upon this 

 animal, it is probably referable to the carbonic acid gas, which exists in fresh 

 water in a much greater proportion than in sea water, and is destructive of 

 the life of all animals. The Nereides, in common with all the other Annel- 

 lida, or red-blooded worms, respire the air contained in the water in which 

 they live, b}' means of a respiratory apparatus, intermediate in its nature 

 between the gills of fishes, and the spiracula and tracheae, or breathing 

 pores and air vessels, of insects. The cavities in which their blood is ex- 

 posed to. the influence of the air admit the water and air by openings in the 

 sides of the animal. Hence, when fresh water, containing carbonic acid gas, 

 is sprinkled upon them, or when they are immersed in it, the gas excites a 

 spasmodic action by its deleterious effect on the animal economy, when 

 introduced, as in this case it probably is, into the circulation. If Dr. Drum- 

 mond, as is probable, used spring water, or rain water, which usually con- 

 tain more carbonic acid gas than river-water, this effect would the more 

 readily follow. The circumstance that the mouth extremity of the Nerei* 

 caeriilea was more sensible to the touch of the poison than any other part, 

 as mentioned in p. 125., is confirmatory of the foregoing explanation; for 

 there is a more direct and speedy communication with the air cells at the 

 anterior part of the animal. It would be interesting if Dr. Drummond 

 would compare the effect, on the animals, of water recently boiled for a con- 

 siderable time, so as to expel the gaseous matter it contains, and also that 

 of carbonic acid gas itselt^ with those of ordinary fresh water and atmo- 

 spheric air. 



Dr. Drummond's remarks on the deleterious effects of fresh water on the 

 marine plants he mentions, as indicative of the impossibility of their exist- 

 ence in lakes and rivers, or other collections of fresh water, are open to the 

 same objection as applies to his inference respecting the Nereis caerulea. 

 Whether the carbonic acid in fresh water is also the cause of the alteration 

 they undergo when placed in it, it would be unsafe to surmise, without 

 direct experiment. — E. W. B. May 7. 1829. 



The Water Shrew. — Sir, Until I read your last Number, I was not aware 

 that the Water Shrew (*S'drex fodiens) had been supposed to have been 

 lost, or was thought an uncommon animal in England. I have frequently 

 met with them ; and in a neglected fish-pond at Risby, a few miles from 

 this, they are very commonly to be seen diving from old stumps, which 

 rise a little above the surface, and returning again, and then reimmersing 

 themselves so quickly as almost to make one doubtful of the cause of their 

 return to the stump ; but having repeatedly noticed them whilst I was 

 angling, I observed a quick nibbling motion of the jaws for the few moments 

 they were out of the water, I am therefore satisfied that those momentary 

 returns were to masticate the food procured by diving. The one seen by 

 Mr. Dovaston seems to have been remarkably shy ; those at Risby, however, 

 are very bold little animals. On the 27th of June last, one continued its 

 search for food within a yard of me for about ten minutes, during which 

 it several times dived and returned five times in a minute, and then, whilst 

 resting a few moments longer than usual, turned and looked full at me ; then 

 sat nearly upright, placing one foot behind the ear, where it smoothed the 

 fur as cats do, and then commenced diving again. I endeavoured to dis- 

 cover what was the food it found, but it was so minute, I was unable to do 

 so. Unlike Mr. Dovaston's specimens, those at Risby feed in the middle of 



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