19:2 Dr. T. Williams on the Mechanism nf 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 tracheae 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 of the tracheal 

 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 different ])arts : 1. the spiral trachea, 

 and 2. its capillary continuation, the membranous. The "former 

 is a continuously tapering tube, branching arborcscently, 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 pei-suasive to satisfy the physiologist that between it and 

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

 simple experiment is quite enough to effect the demolition of M. 

 Blanchard's theory. All structui'es 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-proj)er of the blood-channels. They are 

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

 the tube is under the reagcucy 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 ap])ear with as perfect clearness 

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

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

 difference 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 



