240 PEOFESSOR F. M. BALFOUR. 



and further investigation proved that the tracheae actually 

 started from the slightly swollen inner extremity of the narrow 

 part of the pit, the expanded Avails of the pit forming an 

 umbrella-like covering for the diverging bundles of trachese. 



I have, in fig. 30, attempted to make clear this relation 

 between the expanded Avails of the tracheal pits and th(? 

 trachese. In longitudinal sections of the trunk the trachea, 

 pits do not exhibit the lateral expansion Avhich I have just 

 described^ Avhich proves that the divergence of the bundles of 

 tracheae only takes place laterally and not in an antero-posterior 

 direction. Cells similar in general character to those of the 

 Avails of the tracheal pits are placed between the branches of 

 trachese and someAvhat similar cells, though generally Avith 

 more elongated nuclei, accompany the bundles of trachese as far 

 as they can be followed in my sections. The structure of 

 these parts in the adult Avould, in fact, lead one to suppose 

 that the tracheae had originated at the expense of the cells of 

 pits of the epidermis, and that the cells accompanying the 

 bundles of trachese Avere the remains of cords of cells Avhich 

 sprouted out from the blind ends of the epidermis pits and 

 gave rise in the first instance to the trachese. 



The trachese themselves are extremely minute, unbranched 

 (so far as I could foUoAv them) tubes. Each opening by a 

 separate aperture into the base of the tracheal pit, and measuring 

 about 0"002 mm. in diameter. They exhibit a faint transverse 

 striation Avhich I take to be the indication of a spiral fibre 

 [Moseley {' Phil. Trans.,' 1874, PL 73, fig. 1) states that the 

 trachese branch, but only exceptionally.] 



Situation of the tracheal apertures. — Moseley states 

 (No. 13) that the trachese arise from the skin all over the surface 

 of the body, but are especially developed in certain regions. He 

 finds "a roAV of minute oval openings on the ventral surface of 

 the body," the openings being "situate Avith tolerable regu- 

 larity in the centres of the interspaces betAveen the pairs of 

 members, but additional ones occurring at irregular intervals. 

 Other similar openings occur in depressions on the inner side 

 of the conical foot protuberance." It is difficult in preserved 



