TEACHER OF ARTHEOPODA. 



287 



same number of them, and in some cases they are found on every 



segment of the body. Eich stigma is an ellipse-shaped cleft, sur- 

 rounded by a circular thickening 



of the external chitinous skeleton, 



which can be opened or closed by 



valves. Special muscles close these 



clefts. Each trunk from the stigma 



is lost, sooner or later, in a tuft of 



smaller branches, from which finer 



branches arise, which surround the 



organs. The way in which these 



branch, as well as the length and 

 strength of the branches, varies 

 greatly. Separate tracheal trunks 

 may unite with one another, and 

 form a system of tubes which passes 

 longitudinally along, or transversely 

 across the body, and from which 

 finer ramifications are given off. 



Owing to the distribution of 

 these tracheae throughout the body 

 there is a great difference between 

 the respiratory characters of the 

 Tracheata and of the Branchiata. 

 The medium which is to be respired 

 is distributed through the whole 

 organism ; it is not only the blood- 

 fluid which everywhere bathes the 

 tracheae that can exchange its gases, 



but in the tissues themselves respiration can be effected, for the 

 tracheae are distributed to them, and, indeed, may even come into 

 relation with their form-elements (cf. Fig. 144, tr). But this does 

 not apply to all cases, for, when the tracheae are reduced, the 

 respiratory regions are diminished in number and extent ; in this 

 way the diffuse respiration is localised. In these cases, as when 

 there are branchiae, the blood has to seek out the respiratory organs. 

 In this way the characters of the tracheae influence the circulation. 

 In addition to their respiratory function, the system of tubes filled 

 with air serves to diminish the specific gravity of the body, and is 

 just as important in this relation to Insects during their aquatic 

 stages, as to those Insects which rejoice in wings, and which are 

 able by special arrangements to increase or diminish the amount 

 of air in their tracheal system. 



Fig. 119. Piece of a trachea of a 

 Caterpillar, with its branches, B C D. 

 aEpithelial-like cellular la ver. 6 Nuclei. 



§ 223. 



The arrangement of the tracheal system varies considerably, but 

 all its forms may be derived from that simpler one mentioned above, 

 in which there is a pair of tufted branched tracheae in each metamere. 



