2355. Even in the acephalous bivalve Mollusca and Snails the respiratory openings are almost always situated in the proximity of the anus; their respiration is still a sexual respiration.
2356. For the first time in Insects it becomes a respiration of the trunk; and for the first time in the higher animals a truly animal, namely, a cephalic respiration.
2357. The urinary system is a double system; it conjoins in itself the two highest galvanic processes, that of secretion and excretion.
2358. Secretion is an hepatic character, excretion a pulmonic character. Secretion belongs to nutrition, excretion to respiration. Excretion is an exspiration, secretion an influx or infusion. Secretion is related to excretion as water is to air, as liver to lung, as basis to oxygen.
2359. Secretion takes place, e. g. of bile and saliva, in so far as the processes of the body, especially those of the digestion, are promoted. Excretion is only effected in so far as that the organs, into which what is secreted enters, may obtain a tracheal signification, i. e. evaporation. All excretory orifices are in a certain sense larynges or tracheal orifices. Thus, this relation between secretion and excretion would have been discovered without recourse to conjecture being had upon our part.
2360. The urine is "par excellence" a double product of this kind. It is secreted in the kidneys for simply one object, like the bile. It is excreted, because it belongs to the sexual system, which is essentially exsecernent.
2361. The object or purpose of the urine has not been subverted in all animals. In Birds, where the urinary cyst and intestine are confluent, the urine enters like the bile into the intestine, at least at a spot where there is intestinal matter, which it renders fluid.
2362. In Insects and Snails it appears to invest as a mucus the ova, and serve for their attachment. The same appears to hold good of the spider's web. What has been called purple juice and ink (in the Cephalopods) ranks probably in the signification of urine.
c. SEXUAL INTESTINE.
2363. The sexual intestine is the colon or large intestine, which, in this respect, belongs to the sexual system, as has been indicated at § 2171.