Respiration in Invertebrate Animals. 193 



seems as if a membrane distinct from the two former united 

 together tubularly the coils of the spiral, and that the spiral 

 itself consisted of a hollow tube formed out of cells arranged 

 spirally in the substance of this membrane. In a short 

 time the fibre of the spiral, after immersion in acetic acid or 

 turpentine, loses its dark, highly refractive solid character. It 

 appears distinctly as though its substance were permeated by 

 the fluid and that air was displaced. In this state the spiral 

 looks like a pellucid diaphanous coil wound around the axis 

 formed by the internal membrane. This description applies to 

 every spiral trachea in the body. But there is a limit, different in 

 different structures, at which the spiral ceases. It is at this point 

 that the second division of these tubes or membranous trachece 

 begins. It is not the external covering, as stated by Mr. Newport, 

 which ceases, but the spiral (PI. IX,fig.5,C,e). This fact admits of 

 various and unquestionable proofs. The spiral grows less and less 

 visible until it graduates insensibly into a continuous tube {f,g). 

 It still however retains the peculiar optical character of a trachea. 

 Its edges are faint reddish, from the iridescent decomposition of 

 the light. This appearance was observed by MM. Alessandrini, 

 Filippi and Bassi. The latter of these observers believed that 

 the tint was due to the colour of the hollow cylinder of coloured 

 blood w^iich embraced the aii--tube, corroborative of the views of 

 M. Blanchard. It should rarely be ascribed first to the high 

 refractive index of the air contained in the tube, and next to the 

 density of the external fibrous membrane. It is a character by 

 which a membranous air-tube, though of capillary diameter, can 

 be distinguished with certainty from a blood-channel or a capil- 

 lary blood-vessel. The direct continuity of the bore, as well as 

 of the walls of this membranous capillary air- tube, with the larger 

 and spiral trachea, can be proved in several modes beyond the 

 possibility of dispute. By pressure, skilfully managed, while the 

 specimen is under the microscope, air may be forced from the 

 ' spiral ' into the membranous tube ; but the continuity of the 

 walls of the latter with those of the former is so clear and con- 

 vincing under the microscope, that no other evidence is required 

 to prove that the capillary membranous tube is to the ' spiral' as 

 a capillaiy is to an artery. The diameter of the ' spiral ' trachea 

 constantly deci-eases as it divides ; that of the membranous ob- 

 serves throughout its entire course, whether it multiply into a 

 network (PI. X.fig.9,c?,e), or wavy brushes (fig. 12, c, d,f), or into 

 the muriform plexus which exists in the substance of muscles 

 (PI. X. fig. 15), a uniformity which can compare only with that 

 of true blood-capillaries of the vertebrated anim&l. 



A tracheal tube, in many instances at the point of penetrating 

 into the substance of a solid organ provided with a membranous 



Ann. ^- Mag. N. Hist. Ser.2. F«/. xiii. 13 



