It should be observed that in Peripatus, which does not possess urinary tubes, the segmental organs or nephridia are well developed, hence the tracheal tubes coexisting with them cannot be their homologues. We are therefore compelled to regard the tracheal system as of independent origin, arising in the earliest terrestrial air-breathing arthropod, and not indebted for its origin to any structure found in worms, unless perhaps, as both Kennell and Lang suggest, to dermal glands, since, according to Kennell, certain Hirudinea and many Turbellarian worms possess long, mostly unicellular, glands which spread far through the parenchyma of the body. (Kennell.)
Thus Kennell supposes that the ancestors of the Tracheates had spiracles on every segment of the body where the internal organization allowed them to exist. “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 larvæ 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 segments to form body-regions, and the arrangement and great development of the muscles in the head and thorax, etc. (p. 29.)
Fig. 407.—Section through a tracheal pit and diverging bundles of tracheal tubes taken transversely to the long axis of the body: tr, tracheæ, showing rudimentary spiral fibre; tr. c, cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheæ; tr. o, tracheal stigma; tr. p, tracheal pit.—After Balfour, from Sedgwick.
Kennell has suggested the origin of the tracheæ of Peripatus from the unicellular dermal glands of annelidan ancestors, since he has found glands in certain land-leaches of tropical America, which are provided with enormously long tubular 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 einige Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may add that Carrière supposes from his study of the embryology of the wall-bee (Chalicodoma muraria), published in 1890, that not only the salivary glands, but also the tentorium, are homologues of the tracheæ, while other structures than tracheæ 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 tracheæ of Arachnida from the gills of Limulus; hence if those of Arachnida arose from quite different and more specialized organs than dermal glands, it is not impossible that the tracheæ of Peripatus, Myriopods, and insects arose de novo, and then we need not look for any primitive structures in worms from which they arose.
Although Bütschli 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 tracheæ and urinary tubes as homodynamous 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 urinary 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 tracheæ “have nothing to do with the segmental organs,” as Peripatus besides nephridia possesses both coxal glands and tracheæ.
Both Kennell and Lang derive the coxal glands of Arthropoda from the setiparous or parapodial glands of annelid worms, and the recent endeavor of Bernard to show that the tracheæ arose from setiparous glands seems to be disproved by the fact that in insects as well as in other Arthropoda coxal glands with their outlets exist in the same segments as those bearing stigmata. Reasoning by exclusion, we are led to regard Kennell’s original view as the soundest.
Patten, however, regards the tracheæ as modified ends of nephridia, remarking: “Since in Acilius some of the abdominal tracheæ at first communicate with the cavities of the mesoblastic somites, it is probable that all the tracheæ represent the ectodermic portions of the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)
It is probable, therefore, that the tracheæ first arose as modifications of dermal glands, as in mites and Peripatus, and that at first they were not provided with tænidia (as in Chilopoda), while in later forms tænidia were developed. In the earliest tracheate forms the stigmata were not segmentally arranged, probably appearing irregularly anywhere in the body, but afterwards in the myriopods and insects became serially arranged.