Note 279 — LETTER XXXVIII.
Reading edition
Professor Kidd (Philos. Trans. 1825. 235.) conjectures that the tracheæ, as well as air-vessels, may possibly be blood-vessels; but this hypothesis is inconsistent with the fact recently discovered by Dr. Carus, of a circulation, by other means, in larvæ. See Carus Introd. to Comp. Anat. &c. ii. 400.