COOKE: ON THE HABITAT AND FOOD OF HELIX DESERTORUM. 75 



tions of carbon, in considerable quantity ; also nitrogen, though in smaller 

 amount, and, in all probability, hydrogen. Doubtless these elements were 

 originally united to form complex organic substance. 



"Conjectures as to the sources fn.m which the snails obtained these 

 elements must necessarily be vague and uncertain; from some sort of plant, 

 I should fancy . The presence of carbon and hydrogen, als ) the traces of 

 the metals iron and calcium, all point to this ; but it must be of unusual 

 nature, otherwise potassium ought to occur in recognisable quantity." 



It is natural to conjecture that the snail feeds on some minute form of 

 lichen, which it rasps off the surface of the stones No lichen, however, is 

 visible, and one would naturally expect, even if so low a form of vegetable 

 life as lichen were the staple of food, to find considerable traces of potas- 

 sium. In reference to this, Mr. A. W. Hill, Lecturer in Botany at the 

 University of Cambridge, writes to me : " There is no doubt that small 

 lichens contain potassium salts like all other plants. But I am not sure 

 whether any direct analysis has been made. I have talked to our physiolo- 

 gical botanists, and they agree with the above remarks." 



xMy friend, Mr C. T. Heycock, F.R.S., suggests the possibility that 

 some small alga like Nodoc may be the food. " Nostoc seems to be present 

 in very arid places ; it is invisible by day, but swells into a jelly-like mass 

 when moist, say, with morning dew." 



I am inclined to think that the true solution of the problem lies in the 

 direction suggested by Mr. Heycock. Whether any chemical analysis has 

 as yet been made of this class of alga I am unable to say, but if such 

 analysis were to show that potassium did not occur, or occurred only in 

 excessively small quantities, in this alga, some advance would have been 

 made. One or two points at least are clear. The snail obviously swallows 

 sand in large (quantities. Some of the grains evacuated approach a milli- 

 metre in length and breadth. Its food, therefore, would seem to occur, not 

 on the rocks or stones, but amongst the sand itself, and this will account for 

 the fact that so many specimens are found on the sand. Evidently, too, the 

 food must be of a n iture so minute as to make it impossible to swallow it 

 without swallowing the sand at the same time. Thus the method of nutri- 

 tion in Helix desttrturum will appear to be somewhat analagous to that of the 

 ordinary bivalve mollusc, which nourishes itself on the minute organisms 

 present in the water which passes through its system. 



I may add that 1 have sent a good series of //. di'si-rtoruiii to the 

 British Museum, and also suggested to Mr. E. A. .Smith that he might like 

 to try over again the well-known experiment on the creature's vitality. He 

 kindly consented, and I sent him half-a-dozen good living si)ecimens, which 

 have, under his care, started in an attempt to " beat the record " of the 

 celebrated snail of 1850, which was discovered to be alive after it had been 

 fixed to a tablet for four years (Ann. Mag. Nat. Hist., 1850 (s. 2), vol. vi, 

 p. 68.) 



