sf^;- NOTES ON ARACHNIDS. 523 



the structures are homologous. Now in the " Treatise " they 

 have no reference-letters, and it might be inferred that Balfour, 

 though he saw and figured rudimentary antennae, failed to recognise 

 their importance. But if we turn to his original paper (4), from 

 which the figure in question was taken, we find that he describes 

 these structures, without doubt, as the ectodermal thickenings from 

 which are developed the ganglia innervating the chelicerae. Is it 

 possible, then, that Jaworowski has mistaken these thickenings for 

 appendages, and that the supposed antennae are in reality only 

 incipient nervous centres ? The fact that they sink into cavities 

 seems to lend support to such a view, but their pointed appearance 

 (they are more pointed than in Balfour's figure) suggests appendages. 

 Probably we shall be correct in assuming that the antennae of the 

 ancestors of the Arachnids arose in the region where the ganglia 

 were being developed, and that in some forms, as Tvochosa, the 

 pointed rudiments indicate this, though their subsequent fate is to 

 form part of the nervous centre. 



But, whatever may be the exact meaning of these rudiments, the 

 presence of three segments between the chelicerae and the mouth 

 seems certain. And so, while Jaworowski supports Cholodkowsky's 

 view of the post-oral origin of insect-antennae, it is impossible to 

 regard them as equivalent to the chelicerae of spiders ; for the antennae 

 are said to arise from the first post-oral segment, while the chelicerae 

 are developed from the fourth. The presence of certain transitory 

 appendages in front of the mandibles in insect-embryos, taken in 

 connection with the segments observed by Jaworowski in front of the 

 chelicerae in the embryo of Tvochosa, confirms the view that the man- 

 dibles in the one class correspond with the chelicerae in the other ; 

 and working backwards along the appendages, we identify the 

 maxillae of Arachnids with those of Insects, the first pair of legs in 

 Arachnids with the second pair of maxillae, which form the lower lip 

 of Insects, and the second, third, and fourth pairs of legs in Arachnids 

 with the three pairs of insect-limbs. We conceive that the two 

 classes have diverged from a common stock ; in the Arachnids the 

 antennae have been lost, the head has undergone fusion with the 

 thorax, and two pairs of limbs only have the function of jaws ; while 

 in the Insects the head has been highly specialised, the antennae pre- 

 served, and three pairs of limbs pressed into the service of the mouth. 

 The passage between a walking-limb and a jaw is well shown in the 

 harvestmen (Phalangidea), in which the first two pairs of legs have 

 at their bases small masticatory plates. It is highly interesting to 

 note how, in various groups of Arachnids, the place of antennae is 

 supplied by other limbs ; in the spiders by the maxillary palps or by 

 either of the front two pairs of legs, in the spider-scorpions (Pedipalpi) 

 by the wonderfully attenuated first pair of legs. 



When we consider that the insect and arachnid types were 

 already developed in the Silurian period, we can hardly expect that 



