193 Dr. T. Williams on the Mechanism of Aquatic 



blood or circulatory conduit in its distal ; the microscope every- 

 where proves such a view to be an unmitigated error. It is 

 quite another and more rational supposition to maintain that the 

 passages which bear the trachese may probably serve also to 

 convey the nutritive fluids. Even this opinion requires the 

 evidence of new demonstration. The author will now proceed to 

 consider the results of his own recent investigation, distributed 

 under the following heads: — 1. The structure o-f the tracheae. 

 2. Their distribution ; [a.) in the adult and larval internal struc- 

 tures; {b.) in the branchiae, in connexion with the question 

 of insect aquatic respiration. 3. The anatomical relation in 

 which the tracheae stand to the nutritive fluids. 4. The me- 

 chanism of respiration in air-breathing Articulata. 



^^ Structure of the Trachece, 



The air-tubes in Myriapods, Insects, and Arachnids, admit of 

 division into two distinctly diff'erent parts : I. the spiral trachea, 

 and 2- its capillary continuation, the membranous. The 'former 

 is a continuously tapering tube, branching arborescently, the 

 branches never re-entering. It is always and everywhere fur- 

 nished with an elastic spiral by which its bore is maintained in 

 an open state. It is composed, as stated originally by Sprengel, 

 of three anatomical elements (PI. X. fig. 13) ; the outermost {a) 

 consists of a dense membrane which swells under the agency of 

 acetic acid, and separates from the spiral on which it normally 

 rests and to which it forms a close investment. When raised by 

 acetic acid it retains the impress of the spiral. This would not be 

 the case if the membrane did not naturally closely embrace the 

 spiral. And if it did closely embrace the spiral, it required no 

 further persuasive to satisfy the physiologist that between it ajid 

 the spiral there can by possibility travel no current of blood- This 

 simple experiment is quite enough to eff'ect the demolition of M. 

 Blanchard's theory. All structures external to this membrane 

 belong to the blood-channels (fig. 10, b, c) and not to the trachea. 

 From the coverings of the latter they are quite dissimilar in ana- 

 tomical structure ; they are really the loose delicate membranes 

 which constitute the walls-proper of the blood-channels. They are 

 attached to the trachede only by loose adhesions. If now, while 

 the tube is under the reagency of acetic acid, the eye search for 

 the internal lining membrane (fig. 13, c), which lies on a plane to 

 the inside of the spiral, it will appear with as perfect clearness 

 as the external. It swells and separates from the spiral like 

 the external. It is impossible to p)'^ove the existence of much 

 diff'erence of structure between this and the external membrane ; 

 it is more delicate and less refractive. The spiral lies in the 

 space between these two membranes. On close inspection it 



