DR DAVY ON THE URINARY SECRETION OF FISHES. 547 



receptacle for the secretion but the cloaca, we find it in consistence analogous to 

 that of birds, snakes, and lizards, a soft solid ; in insects, as far as my observa- 

 tions have extended, and they have been numerous,* it is composed chiefly of an 

 alkaline lithate ; but in the others, the spiders and scorpions, of guanine.f 



Of the secretion in the mollusca, also without a urinary bladder, I can ven- 

 ture to say little. In two instances I have found it to be lithic acid ; the indivi- 

 duals in the excrement of which I detected this compound were our common 

 slug (Lima agrestis), and the large snail of Tobago (Helix oblong a f). 



Of animals lower in the organic scale, the only ones I have examined with 

 any positive result have been two of the Myriapoda, — the common centipede of 

 the West Indies (Scolopendra morsitans), and our millipede (lulus terrestris), the 

 one voracious, feeding on insects, the other feeding on vegetable matter. In the 

 mixed excrement of the scolopendra, lithate of ammonia in abundance was de- 

 tected \\ but in that of the millipede, merely a trace of lithic acid. 



In this brief notice of the urinary secretion in the several classes of animals 

 mentioned, I have, as I premised, taken notice only of its principal ingredient ; I 

 would further beg to remark, that in stating that the quality of the secretion is 

 independent of the quality of the food, I would wish to be understood as not 

 holding the opinion that it is not in some measure modified by the kind of food, — 

 especially as regards the quantity of matter eliminated. As might be expected, 

 the larger the proportion of nitrogen in the food consumed, the larger, cceteris pa- 

 ribus, seems to be the quantity of the nitrogenous compound excreted, and vice 

 versa. Moreover, when the food is entirely vegetable, there seems to be in some 

 instances a tendency towards the production of the hippuric acid rather than of 

 the lithic. MM. Magnon and Lehmann have found this compound in the urine of 

 the tortoise feeding on lettuce ;§ and have found it mixed with lithic acid in the 

 urine of caterpillars feeding exclusively on vegetables, — a result which accords 

 with my own experience. 



In the animal economy we see commonly, amongst the different classes of 

 animals, a certain relation and accordance of functions conducive in action to 

 the elaboration and wellbeing of each individual structure. Such a relation is 

 manifest between the kidneys and the lungs ; the former the depurator of nitro- 



* Trans. Ent. Society, vol. iii., N. S. 



f When I first examined the excrement of spiders and scorpions in 1847-1848, operating on 

 minute quantities, I inferred that it consisted chiefly of xanthic oxide : Guanine was not then known. 

 Since its discovery by Bodo Unger, I have re-examined portions of the excrement of each, which I 

 brought from the West Indies, and have satisfied myself that the principal ingredient of both is this 

 compound ; I have also found it, in accordance with the researches of Will and Gorup-Besanez, 

 to form the chief portion of the excrement of our spiders. The very low degree in which this excre- 

 ment is soluble in cold muriatic acid may account for its having been first confounded with the 

 xanthic oxide. 



+ Edin. Phil. Jour., vol. xlv. p. 383. 



§ Lehmann's Physiological Chemistry, vol. ii., p. 458. 



VOL. XXI. PART IV. 7 H 



