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. 
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 oblonga ?). 
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 (Zulus 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, ceteris 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. Maenon and Lenmann 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. ii., N. 8. 
{ 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 Bopo Unazr, 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 Witt and Gorup-Busanxz, 
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. 
t Edin. Phil. Jour., vol. xlv. p. 383. 
§ Leumann’s Physiological Chemistry, vol. ii., p. 458. 
VOL. XXI. PART Iv. 7H 
