896 ANATOMY AND DEVELOPMENT 



towards the lumen of the pit by a very delicate cuticular mem- 

 brane continuous with the cuticle covering the surface of the 

 body. The pits vary somewhat in depth; the pit figured was 

 about O'OQ mm. It perforates the dermis and terminates in the 

 subjacent muscular layer. The investigation of the inner end of 

 the pit gave me some little trouble. 



Transverse sections (fig. 30) through the trunk containing a 

 tracheal opening shew that the walls of the pit expanded inter- 

 nally in a mushroom-like fashion, the narrow part being, how- 

 ever, often excentric in relation to the centre of the expanded 

 part. 



Although it was clear that the tracheae started from the ex- 

 panded region of the walls of the pit, I could not find that the 

 lumen of the pit dilated into a large vesicle in this part, and 

 further investigation proved that the tracheae actually started 

 from the slightly swollen inner extremity of the narrow part of 

 the pit, the expanded walls of the pit forming an umbrella-like 

 covering for the diverging bundles of tracheae. 



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

 tween the expanded walls of the tracheal pits and the tracheae. 

 In longitudinal sections of the trunk the tracheal pits do not 

 exhibit the lateral expansion which I have just described, which 

 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 walls of the tracheal 

 pits are placed between the branches of tracheae, and somewhat 

 similar cells, though generally with more elongated nuclei, ac- 

 company the bundles of tracheae as far as they can be followed 

 in my sections. The structure of these parts in the adult would, 

 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 tracheae were the remains of 

 cords of cells which sprouted out from the blind ends of the 

 epidermis pits and gave rise in the first instance to the tracheae. 



The tracheae themselves are extremely minute, unbranched 

 (so far as I could follow them) tubes. Each opening by a sepa- 

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

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

 striation, which I take to be the indication of a spiral fibre. 



