ORIGIN OF THE TRACHEA 



443 



"The reduction of the breathing holes to a smaller number, and their restriction 

 of a pair only to a single segment, was brought about partly by adaptation to a 

 peculiar mode of life, as insect larvae especially teach us, partly also I may 

 say mechanically as a result of the obstruction to their development made by 

 the growth or excessive development of other organs." Among these he reckons 

 the thick, dense cuticula of the integument, the internal fusion of several seg- 

 ments to form body-regions, and the arrangement and great development of the 

 muscles in the head and thorax, etc. (p. 29.) 



Kennell has suggested the origin of the tracheae of Peripatus from the unicellu- 

 lar dermal glands of annelidan ancestors, since he has found glands in certain 

 land-reaches of tropical America, which are provided with enormously long tubu- 

 lar passages united into bundles and opening externally, these tubes appearing 

 to be slightly chitinized. Fig. 407 will show the appearance of a bundle of fine 

 tracheal tubes of Peripatus ending at the bottom of a follicle formed by a deep 

 invagination of the integument, which may be regarded as a primitive spiracle. 

 (See Kennell, Ueber eiuige Landblutegel des tropical America, Zool. Jahrb. ii, 

 1886 ; also Die Verwandtschaftsverhaltnisse derArthropoden, 1891, p. 25.) We 

 may add that Carriere supposes from his study of the embryology of the wall-bee 

 (Chalicodoma nmraria}, published in 1890, that not only the salivary glands, but 

 also the tentorium, are homologues of the trachese, while other structures than 

 tracheae may have evolved from 

 unicellular dermal glands, which 

 are widely distributed. It may 

 in this connection be observed 

 that some authors derive the 

 book-lungs or book-leaf tracheae 

 of Arachuida from the gills of 

 Limulus ; hence if those of 

 Arachnida arose from quite dif- 

 ferent and more specialized 

 organs than dermal glands, it is 

 not impossible that the tracheae 

 of Peripatus, Myriopods, and 

 insects arose de ?tovo, and then 

 we need not look for any primi- 

 tive structures in worms from 

 which they arose. 



Although Biitschli in 1870 in 

 his embryology of the honey-bee 

 called attention to the "great 

 similarity which the eleven pairs 

 of invaginations in the eleven 



first trunk-segments in their first indication (anlage) have with the spinning- 

 glands, and also with the segmental organs of Annelids," he did not go further 

 than this, and it is now known that in the 2d maxillary segment open not only 

 spinning-glands, but in the embryo a pair of stigmata. 



Paul Mayer, however, regarded the tracheae and urinary tubes as homodyna- 

 mous structures, and this view was advocated by Grassi (1885) for the reason 

 that while in the embryo honey-bee there are ten pairs of stigmata, the first 

 thoracic and two last abdominal segments wanting them, the germs of the uri- 

 nary tubes arise in a corresponding situation on the two last abdominal segments. 

 To this view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus the 

 nephridia and tracheae " have nothing to do with the segmental organs," as Peri- 

 patus besides nephridia possesses both coxal glands and tracheae. 



^. br.o. 



FIG. 407. Section through a tracheal pit and diverg- 

 ing: bundles of tracheal tubes taken transversely to the 

 Ion? axis of the body : tr, trachea-, showing rudimentary 

 spiral fibre ; tr. c, cells resembling- those lining- the traeheal 

 pits, which occur at intervals along- the course of the 

 trachea- ; tr. , tracheal stigma; tr. p, tracheal pit. 

 After Balfour, from Sedgwick. 



