GENERAL CONSIDERATIONS REGARDING THE ARACIINIDA. Ill 



does not appear to us important, since we attach no great value 

 to this comparison of the Arachmda with the Insecta, and seek for 

 the relationships of the former not so much in the domain of the 

 "Tracheata" as among the branchiate forms, viz., the Xiphosura, 

 as Ray Lankester and others have also done. "We are, therefore, 

 inclined to agree with those zoologists who consider the Arachmda 

 and the other air-breathing Arthropoda as two distinct series, and also 

 assume a separate origin for the tracheae in these two divisions. 

 The agreement existing between the organisation of the Arachnida 

 and that of the Xiphosura compels us to adopt this view. 



We have already pointed out the agreement in outer structure 

 between the Scorpiones and Limidus (Vol. ii., p. 357), especially in 

 the numbers of the segments and limbs. In Limulus, as in the 

 Arachnida, we find six pairs of limbs on the cephalo-thorax, so that 

 a homology is suggested. "We have just compared the first pair 

 of limbs, the chelicerae, to the second antennae of the Crustacea, 

 chiefly because the ganglia of these limbs, which arise post-orally, 

 become united with the supra-oesophageal ganglion, as is the case 

 also with the second antennae in the Crustacea (Vol ii., p. 164), and 

 this process gains in significance when it is found repeated in the 

 maxillary ganglia of Peripatus (p. 193). Xo such process is to be 

 found in the Insecta, and we conclude that the limb in question is 

 wanting in them. 



"We must not neglect to record the fact that, in the Opiliones and the Acarina, 

 the chelicerae are said to he innervated from the thoracic ganglionic mass 

 {Leydig, No. 40, b, and "Winkler, No. 106). A final elucidation of this point 

 is very desirahle. 



The presence in the Araneae of another pair of cephalic limbs 

 besides the two already mentioned has repeatedly been maintained. 

 Two prominences are said to appear in front of the rudiments of the 

 chelicerae and again to disappear (Croneberg, Jaworowski). It 

 was assumed that these conjectural limbs became united with the 

 rostrum (Croneberg, Lendl*), which, according to other observers, 

 was found to have a paired rudiment (Schimkewitsch). There was 

 a general tendency to seek in the rostrum the rudiment of one, or, 

 indeed, perhaps of several pairs of limbs, and it was thought that this 

 could even be proved in the adult animal (Scorpiones, Solifugae, 



* According to Lendl, the vestigial limbs lie between the chelicerae ami 

 the pedipalps, and correspond to the mandibles of the Insecta, while the 

 chelicerae, by their position and their manner of moving, show themselves to be 

 true antennae. The shifting forward of the pedipalps pressed the conjectural 

 mandibles against the rudiment of the upper lip, so as to fuse with it. 



