450 NATURAL SCIENCE. June. 



{Ovgyia antiqtia) recalls an annelid parapodium with its setae. This 

 supposed difference in origin of the breathing-tubes in the two classes 

 leads to a supposed corresponding difference in the origin of the 

 limbs, those of the insects being traced to the ventral series of para- 

 podia of the annelid, and those of the arachnids (and also crusta- 

 teans) to the dorsal series. Ingenious as this theory is, the great 

 cleft which it makes between the insects and arachnids by destroying 

 the homology of their appendages, seems a grave objection. The 

 higher annelids have undergone too considerable a development 

 along their own particular line to be safely regarded as approximating 

 to the extinct ancestors of a distinct phylum. 



However, in a recent memoir, Sinclair (9) also suggests the 

 derivation of lung-sacs from tracheae rather than that of the latter 

 from the former. He describes peculiar breathing-organs in the 

 Scutigeridae, consisting of dorsal slits opening into air-sacs, from each 

 of which a number of tubes, tirranged in two semicircular masses, are 

 given off; these organs are believed to represent a stage between 

 ordinary tracheae and the lung-books of spiders, the lungs of scorpions 

 representing the highest development of the series. 



But the relationship between the various kinds of breathing- 

 organs depends upon the question whether the lung-bearing or 

 tracheate orders of arachnids are the more primitive. In the 

 scorpions, we find four pairs of lung-sacs, and the full segmentation 

 of these animals has already been dwelt upon as evidence of their 

 archaic nature. Moreover, while palaeontology has not yet thrown 

 much detailed light on the history of the arachnid groups, strong 

 confirmation of this view is derived from the fact that remains of 

 scorpions occur in the Silurian rocks, and that these are the oldest 

 air-breathing animals of which we have certain knowledge. In the 

 scorpion-spiders, and in the lower families of the true spiders, two 

 pairs of lung-sacs occur, but in the higher spiders, as already 

 mentioned, the hinder pair of these are replaced by tracheae. Here, 

 again, such knowledge of fossil spiders as we possess supports the 

 view that lung-sacs preceded tracheae, for while four-lunged spiders 

 lived in Carboniferous times, tlie higher members of the group have 

 only been found fossil in Tertiary formations. The comparatively 

 recent orgin of the higher spiders is also suggested by a study of 

 their distribution, most of the families and genera being world-wide 

 in their range. Marx (10) has recently remarked that among 292 

 species of spiders from the Arctic regions, none can be referred to 

 special genera ; and British naturalists who have read Hudson's 

 " Naturalist in La Plata " must have observed that, while the 

 mammals and birds and insects described are far removed, indeed, 

 from our native animals, in the chapter on spiders we are introduced 

 to such familiar friends as Tetragnatha, Zilla, Pholcus, and Argyroneta. 

 Among the four-lunged spiders we meet with the confined and 

 discontinuous distributions so characteristic of old-time groups of 



