On the Mechanism of Aquatic Respiration. 243 XXV. — On the Mechanism of Aquatic Respiration and on the Structure of the Organs of Breathing in Invertebrate Animals. By Thomas Williams, M.D, Lond,, Licentiate of the Royal College of Physicians, formerly Demonstrator on Structural Anatomy at Guy's Hospital, and now of Swansea. [With a Plate.] The mechanism of breathing in the countless hosts of inverte-brate animals which people the ocean, offers a problem which has never yet been satisfactorily solved. The mode in which life is sustained in those degraded forms, in which " a circulation of blood " is not to be discovered, has long stimulated the curious wonder of the naturalist. Fishes and Cetacea excepted, the in-vertebrate animals constitute the entire population of the ocean. Insects excepted, all invertebrate animals are aquatic. Hence the wide range of interest which belongs to this subject. How animals breathe is not second in importance to the question how they live. Every observer studies the latter, few the former. There are " habits " associated with the manner in which the function of breathing is performed which are well-fitted to win admiration. Wanting the knowledge of this process, not the smaller half of the history of an animal remains to be acquired. It is the aim of this memoir to demonstrate first the ana-tomical conditions under which the office of respiration is per-formed in the invertebrate animals, and then to study the pro-cess itself. The anatomical conditions will prove as various as the classes of which this subkingdom is composed. Two pri-mary divisions of this subject demand at once to be recognised; — 1st, that comprising those organs which adapt the animal for atmospheric breathing; 2nd, that qualifying it to respire in water. The latter, embracing varieties more striking and numerous than the former division, should again be resolved into two denominations, of which one would comprehend the me-chanism of those organs by which the chylaqueous fluid is sub-mitted to the agency of the aerating element, and the other, that of those fitted to expose the true blood*. All vertebrated animals, fishes excepted, breathe on the atmo-spheric plan. All invertebrate animals, insects excepted, respire on the aquatic model. The organs used in the first method are more complex than those comprised in the second; while the chylaqueous fluid is subjected to respiration, through the least complexly arranged mechanisms. The simpler the fluid to be * The author would here beg to refer the reader, for a full statement of the grounds of this latter subdivision, to his pa])er on the Blood-proper and Chylaqueous Fluid, &c., in the Phil. Trans., Nov. 1852. 17*
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