294 Dr. T. Williams on the Mechanism of Aquatic narrow and decompound state of D. ligulata, as it occurs on the British coast, to that of the Alga under consideration, is very re-markable ; and as far as I know, intermediate states have not occurred. But Professor J. Agardh speaks of the frond of some French specimens of D. ligulata as an inch in breadth. Professor J. AgardVs var. /3. (Z>. herbacea, Lamx.) and var. 7. [Sporochnus herbaceus, var. firma, Ag. Syst.) do not at all agree in their pin-nated forms and spinuloso-serrate margin with our plant ; and if his conjecture should eventually prove to be correct, it would be difficult to adduce a more extraordinary deviation from a specific type. It might be described as var. 8. subsimplex. In the mean time a figure (PI. XIV. fig. 1) of so interesting an Alga will, it is hoped, be not unacceptable to the British botanist. XXVIII. — 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 two Plates.] [Continued from p. 200.] The epidermal skeleton of the Arthropoda is histologically pe-culiar. Chitine was first defined by Odier*. In the year 1845 it was more fully investigated by C. Schmidt f. By Lassaigne it has been distinguished under the name of Endomaderm : it is a proximate principle which resembles cellulose. Both are inso-luble in caustic potass. Nitrogen however is present in chitine and absent in cellulose : it is the animal basis of the integu-mentary structures of Insects and Crustacea. It is a principle of low vital properties. To the presence of this substance is pro-bably to be ascribed the fact, already mentioned as extraordinary, of the universal absence of vibratile cilia from all the structures, of Insects and Crustacea. And why is vibratility not a pro-perty of those organized parts of which chitine is the proximate basis ? The very definedness of this question marks an advance in the real science of physiology. Eff^ect is linked to its true cause, attribute to its right substratum, function to its imme-diate instrument. Chitine is produced under two distinct con-ditions : in Insects it occurs under the circumstances of atmo-spheric respiration, in Crustacea under those of the aquatic. * Mem. de la Soc. d'Hist. Nat. de Paris, 1823, p. 29. t Zur Vergleich. Physiol, d. Wirbellos. Thiere, p. 32.
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