Respiration in Invertebrate Animals. ' 195 



vertebratcd animal. In the muscles the ultimate membranous 

 tracheae divide and unite plexiformly (figs. 14, 15). The meshes 

 are large and oblong (fig. 15). Some tubes run parallel with the 

 ultimate muscle-hbre ; others cross the latter at right angles, 

 connecting the former. In the glands the capillary tubes enclose 

 the space occupied by the gland-cells (fig. 9) : they unquestion- 

 ably reticulatCj and in their ultimate form preserve a remarkable 

 uniformity of diameter. In many parts of the mucous mem- 

 branes they observe a peculiar wavij method of distribution 

 (fig. 12). They elaborately reticulate in the loose structure be- 

 neath the integuments. It is by no means improbable that M. 

 E. Blanchard has mistaken the plexus formed by the trachece m 

 the integuments of the Arachnids for a rete consisting of true 

 blood-vessels. In the nervous tissue they follow two modes of 

 subdivision. The brain substance is actually penetrated by the 

 plexiform capillary tubes. The nerves are accompanied by long 

 undulating filaments. 



In some of the voluntary muscles the tracheae are profusely 

 numerous. The larger spiral branches enter the sheaths of the 

 muscle = fascicles at right angles (fig. 14) ; the membranous 

 tracheae into which in the substance of the muscle they sub- 

 divide, coincide generally in direction with the fibres of the 

 muscles (fig. 15). In other muscles the primary entering tracheae 

 are few in number. As a rule it seems at present probable, that 

 the volume of air (oxygen) which by means of the tracheae enters 

 into the substance of a solid organ in the tracheary Articulata is 

 directly as the vital importance of that organ. The reticulation 

 of the tracheae is most dense and profuse in the glandular and 

 nervous structures. The large spiral air- tubes which travel along 

 the axes of the spacious blood-channels, detach from their sides 

 here and there minute wavy branches (fig. 10,^', b) which ^oa^ in 

 the fluid, and which appear to be expressly intended to aerate 

 the fluids. These floating air-tubes are everywhere seen where 

 the blood-stream comes into contact with the main trachea. The 

 main tracheae are simply convective. It will afterwards appear 

 that the function of these floating trachecB is distinct from that 

 of those plexiform extremes of the system which penetrate and 

 traverse the substance of the solid organs. These aerate imme- 

 diately the solids, those the fluids. 



The distribution and subdivision of the trachea in the branchiae 

 of the aquatic larvae of Insects involve the consideration of the 

 mechanism and significance of aquatic respiration as it occurs in 

 the young of the air-breathing Articulata. Is it real aquatic 

 breathing, or is it only apparently so ? 



In the larvae and pupae of gnats, the branchiae exist in the form 

 of slender hair-like organs arranged in tufts. Each filament is 



13* 



