562 E. RAY LANKESTER. 



accepted these two conclusions, we formulate the generalisa- 

 tion that trachege can be iudependently acquired by various 

 branches of Arthropod descent in adaptation to a terrestrial 

 as opposed to an aquatic mode of life. A great point of 

 interest, therefore, exists in the knowledge of the structure 

 and embryology of trachete in the different groups. It must 

 be confessed that we have not such full knowledge on this 

 head as could be wished for. Trachea? are essentially tubes 

 like blood-vessels — appai'ently formed from the same tissue 

 elements as blood-vessels — which contain air in place of blood, 

 and usually communicate by definite orifices, the tracheal 

 stigmata, with the atmosphere. They are lined internally by 

 a cuticular deposit of chitin. In Peripatusand the Diplopods 

 they consist of bunches of fine tubes which do not branch, 

 but diverge from one another ; the chitinous lining is smooth. 

 In the Hexapods and Chilopods, and the Arachnids (usually), 

 they form tree-like branching structures, and their finest 

 branches are finer than any blood capillary, actually in some 

 cases penetrating a single cell and supplying it with gaseous 

 oxygen. In these forms the chitinous lining of the tubes is 

 thickened by a close- set spiral ridge similar to the spiral 

 thickening of the cellulose wall of the spiral vessels of plants. 

 It is a noteworthy fact that other tubes in these same terres- 

 trial Arthropoda — namely, the ducts of glands — are similarly 

 strengthened by a chitinous cuticle, and that a spiral or 

 annular thickening of the cuticle is developed in them also. 

 Chitin is not exclusively an ectodermal product, but occurs 

 also in cartilaginous skeletal plates of mesoblastic origin 

 (connective tissue). The immediate cavities or pits into which 

 the tracheal stigmata open appear to be in many cases ecto- 

 dermic in sinkings, but there seems to be no reason (based on 

 embryological observation) for regarding the trachete as an 

 ingrowth of the ectoderm. They appear, in fact, to be an 

 air-holding modification of the vasifactive connective tissue. 

 Tracheae are abundant just in proportion as blood-vessels 

 become suppressed. They are reciprocally exclusive. It 

 seems not improbable that they are two modifications of the 



