166 THE COCKROACH. 



Tartari, near Tivoli, a small lake whose waters are warm and 

 saturated with carbonic acid. Insects abound on its floating 

 islands ; though water birds, attracted by the abundance of 

 food, are obliged to confine themselves to the banks, as the 

 carbonic acid disengaged from the surface would be fatal to 

 them, if they ventured to swim upon it when tranquil. 



Or lorn of Tracheal Respiration. 



Kowalewsky, Butschli, and Hatschek have described the 

 first stages of development of the tracheal system. Lateral 

 pouches form in the integument ; these send out anterior and 

 posterior extensions, which anastomose and form the longi- 

 tudinal trunks. The tracheal ramifications are not formed 

 by a process of direct invagination, but by the separation of 

 chitinogenous cells, which cohere into strings, and then form 

 irregular tubules. The cells secrete a chitinous lining, and 



o O' 



afterwards lose their distinct contours, fusing to a continuous 

 tissue, in which the individual cells are indicated only by their 

 nuclei, though by appropriate re-agents the cell boundaries can 

 be defined. 



The ingenious hypothesis propounded by Gegenbaur, that 

 the tracheal tubes of Insects were originally adapted to aquatic 

 respiration, and that the stigmata arose as the scars of disused 

 tracheal gills, has been discussed in chap. iv. Semper has 

 suggested* that tracheoe may be modified segmental organs, but 

 the most probable view of their origin is that put forth by 

 Moseley,f that they arose as ramified cutaneous glands. In 

 Peripatus the openings are distributed irregularly over the 

 body ; the external orifices lead to pits, from which simple 

 tubes, with but slight spiral markings, extend into the deeper 

 tissues. 



* Arbeiten a. d. Zool. Zoot. lust. Wurzburg. Bd. ]!., 1874. 

 t Phil. Trans., 1874, p. 757. 



