S86 Dr Davy's Observatio?is on (he Centipede. 



consistence, and, as seen under the microscope, composed of 

 globules chiefly mixed with a few epithelium scales. The 

 globules were about 2 lyVo of an inch in diameter. Acted on 

 by nitric acid and heat, I have found them to contain lithic 

 acid, which is probably in the form of lithate of ammonia, 

 and probably also combined with some animal matter. That 

 it is so combined, I infer both from the comparatively large 

 size of the globules, and from the purple colour, the result of 

 the action of the acid and heat, being less intense than in 

 the instance of pure lithate of ammonia similarly tested. A 

 third kind of matter is occasionally met with, of a grey colour 

 or nearly white, which, chemically examined, appears to con- 

 sist chiefly of earthy substances, carbonate of lime and sili- 

 ceous sand ; and which, it may be inferred, has been taken 

 in with the food, either intentionally — instinctively, as the 

 like matter is swallowed by fowls to afford material for the 

 formation of the shell of the egg, — or accidentally, adhering 

 to what is eaten, still serving in part the same purpose. 

 This matter occurs mixed with the dark, olive-green intesti- 

 nal excrement. The white globular matter, containing lithic 

 acid, and which it may be concluded is a secretion of the 

 urinary kind, is not mixed with the intestinal matter, but is 

 merely attached to it, much in the same manner as in the 

 instance of the mixed excrement of the centipede ; but the 

 urinary part is in greatly smaller proportion, the secretion, 

 indeed, is in very minute quantity compared with the excre- 

 tion ; that is, the urinary with the alvine. 



As it is probable, reasoning from analogy, that the excre- 

 ment of all snails and slugs is similar in composition, and 

 contains lithic acid, it may be inferred that their casts are 

 even more useful, considered as a natural manure, than has 

 commonly been supposed ; storing up nitrogen, in lithate of 

 ammonia, and by the slow decomposition of this compound, 

 restoring it, and affording an element of support for growing 

 plants, the nitrogen so stored up being an excess over that 

 required for the growth and sustenance of the animals, and 

 itself probably derived from vegetables. 



As this large snail appeared to me a good subject for mak- 

 ing trials on its temperature, I instituted a few, the results 



