i897. ARE THE ARTHROPODA A NATURAL GROUP? 107 



respiratory surface. The gills are the homologues of the lung lamellae, 

 since it has been established by Kingsley's interesting researches 

 (Quart, jfoiirn. Micr. Sci., 1885) that the limbs of Limulus are protruded 

 from a depression ; and in Ligia, Oniscus, and other Crustacea/ similar 

 invaginations have been observed near the limbs. In all groups of 

 Arthropoda, indeed, in the embryo, limbs are to be found which 

 reveal the primitive type^ (Fig. 2 C). Since, therefore, the Crustacea 

 became aquatic, it is evident that the depressions (sac-tracheae) 

 functioning as respiratory organs must not only have ceased to develop 

 further, but must have degenerated, and the absence of tracheae in 

 this class is thus accounted for. 



The parapodia and gills of the annelids are due to the same 

 evolutionary factors. The annulation of the annelid body and the 

 hardening of the integument at any rate testify to the fact that they 

 also are of terrestrial origin. The original sac-like respiratory organs 

 have disappeared, the parapodia and the branchiae remaining behind 

 as vestiges. 



While referring the reader for details to the original works on the 

 subject, I think that in thus deducing arthropod hmbs from lungs I 

 have disposed of the assumed derivation of the Tracheata from the 

 Branchiata, and have come nearer to the true origin of the 

 Arthropoda, more especially of Peripatus. 



It is, however, difficult to arrive at definite conclusions as to the 

 systematic position of the last-named arthropod. We are as yet only 

 on the threshold of a great building in one corner of which truth 

 dwells. The assumption that Peripatus is most nearly related to the 

 most highly developed annelid, and Mutton's conclusion that the 

 former should be directly classed with the latter are not without 

 justification. It is certain that Peripatus cannot possibly be derived 

 from the Crustacea. I inchne to the view that Peripatus is a 

 secondarily adapted land animal. In the first process of adaptation, 

 it acquired gills and parapodia-hke truncated limbs, which, owing to 

 aquatic life, could not develop further. The animal, in this process, 

 lost its respiratory organs. Later, by gradual re-adaptation, it re- 

 acquired breathing organs in the form of trachea-like invaginations of 

 the integument, sometimes scattered all over the body. This view 

 gains in probability from the fact that other arthropods {Oniscus, 

 Birgus latro) have become readapted to land life. 



When we consider the origin of the limbs and the gills, as well as 

 of that of the parapodia and gills of the annelids, we arrive at the 

 conclusion that water and land have from the first shaped these 

 animals, and that they are all to be derived from worm-like ancestors. 

 In this case, Peripatus represents a transition form between the 

 Arthropoda and the highest annelids, being connected with the former 



icf. Zeitschr. fur wiss. Zool., Bd. Iviii., pp. 54-78; Mitzeum Lembevg, 1896; Zool, 

 Anz., Nos. 455 and 473. 



-cf. also Zool. Anz. Nos. 363 and 364, and especially No. 392, where a case is 

 cited of a limb possessing an appendage as well as the endo-, exo- and epi-podite. 



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