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. Agardh^s var. /3. (D. 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. S. 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. Schmidtf. 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. Efiect 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 Soe. d'Hist. Nat. de Paris, 1823, p. 29. 

 t Zur Vergleich. Physiol, d. Wirbellos. Thiere, p. 32. 



