i897. ^RE THE ARTHROPODA A NATURAL GROUP? 105 



Peripatus I have never examined, but, so far as I can see, it 

 stands apart from all Arthropoda and must be removed from the 

 series. Korschelt and Heider have placed it among these animals, in 

 front of the Myriopoda, and have put together all the essential 

 characters known in 1890 ; but this same chapter of their work shows 

 very plainly the enormous differences between Peripatus and the five 

 classes of Arthropoda, with the structure of which I am tolerably 

 acquainted through studies during many years. Such features as the 

 quality and arrangement of muscles, the series of nephridia, the 

 structure of the mouth and the "limbs," the eyes, and the generative 

 organs with their ducts seem to be sufficient. Furthermore, it will 

 be admitted that Peripatus has nothing to do with the Crustacea, and 

 since Scolopendrella and the Thysanura show a closer relation to the 

 lower Malacostraca than hitherto recognised, while the Myriopoda are 

 divergent, specialised forms (for instance, the "gnathochilarium" is, 

 in spite of embryology, formed by the coalescence of two pairs of 

 mouth-limbs), I think that no place for Peripatus will be found among 

 the Arthropoda. I am most inclined to look upon this animal as a 

 ** worm specially modified for a terrestrial existence," and it must, in 

 all probability, take the rank of a separate class between the multitude 

 of very different forms together composing the "worms" (such as 

 Annulata, Chaetognatha, Sipunculida, Nemertina, etc.) whose mutual 

 relationships are still in part rather obscure. 



H. J. Hansen. 



Copenhagen. 



The conclusion of F. W. Hutton as to the position of Peripatus^ 

 expresses the view which I also hold, and have expressed in Jenaische 

 Zeitschrift, Bd. xxx., p. 67, 1896. 



In a treatise entitled " Die Entwickelung der sog. Lungen bei 

 den Arachniden, &c.," (Zeitschv. wiss. Zool. Bd. Iviii. : See also 

 Muzeum Lemberg, 1896), in connection with the discovery that, in 

 Trochosa, the so-called lung passes into tracheal branches, while in 

 other spiders and in scorpions it is sac-like and thus must actually be 

 regarded as a sac-trachea which does not develop further, I have 

 shown that the view that the Arachnida are to be derived from 

 Crustacea, whose respiratory appendages sank beneath the surface of 

 the body and thus led to the formation of arachnidan lungs, cannot be 

 maintained. On the contrary, anatomy and ontogeny both show that 

 the arthropodan appendages, together with the gills, are to be 

 deduced from the lungs. 



It is self-evident that the Arthropoda are descended from those 

 lower worm-like animals which, in adaptation to terrestrial life, 

 developed integumental depressions, i.e., sac-tracheae, for the purpose 

 of breathing air ; the possibility that this adaptation was of special 

 utiUty to animals dwelling on the shore and subject to the recurrence 

 of the tides is by no means denied. Exposure to air and sun brought 



