GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. 113 



"thoracic" or fourth cephalo-thoracic segment. These peculiarities 

 have led to the Solifugae, which breathe through tracheae, being 

 brought into relation with the Insecta ; but we have already shown 

 (p. 36) that Ave cannot regard these characters of the Solifugae as 

 primitive, nor consider the Solifugae themselves as intermediate 

 forms between the Arachnida and the Insecta. In judging of the 

 relationships of the Solifugae, it is important to note that in them 

 also the chelicerae are innervated from the brain (Weissenborn, 

 No. 16), and are thus proved to be homologous with the chelicerae 

 of other Arachnids. An attempt to compare them with the antennae 

 of the Insecta in order to explain their innervation will hardly be 

 made, their whole development being opposed to this. In making a 

 comparison with the Insecta, we should conclude rather that the 

 antennae, which are to be regarded as a pair of cephalic limbs, are 

 here wanting. 



The abdomen of the Arachnida is characterised chiefly by the great 

 reduction of its segmentation, except in some divisions however, 

 where the segments are very distinct. In the Scorpiones, the 

 posterior part of the body is divided into a pre-abdomen and a 

 post-abdomen, and is of great length. It might, indeed, be con- 

 sidered as doubtful whether the lengthening of this part were not 

 secondary, but for the fact that other Arachnida, during embryonic 

 life, have sometimes this same number of segments, and also show 

 indications of the division into pre- and post-abdomen (Araneae, 

 pp. 50 and 57). 



In the fossil Xiphosura (Hemiaspis, Belinums), as well as in the 

 Gigantostraca, the number of abdominal segments is larger than 

 in Limulus, this makes it very probable that the abdomen of the 

 latter has arisen through the fusion of a number of post-abdominal 

 segments, and is thus homologous with the post-abdomen of the 

 Scorpiones (Vol. ii., p. 358). The latter thus show, in the retention 

 of the richly-segmented abdomen and in their segmentation generally, 

 a very primitive character. It has been conjectured that the length 

 and mobility of the abdomen are connected Avith the poison-sting 

 which arms its extremity, and which is thus the more easily brought 

 into use (Weissenborn). 



Great concentration of the organs is evident in the Arachnida, 

 and the further forms are removed from those which we may rightly 

 consider as the most primitive, the greater is the degeneration found 

 in them, this degeneration reaching its highest degree in the Acarina. 

 The derived forms of the Arachnida are thus simpler in their 



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