TRACHEATA. 455 



It deserves to be noted that, in both Scorpio and the Spider, 

 the chelicerae are borne in the embryo by the first post-oral 

 segment, and provided with a distinct ganglion, so that they 

 cannot correspond (as they are usually supposed to do) with the 

 antennae of Insects, which are always developed on the prae-oral 

 lobes, and never supplied by an independent ganglion. 



The chelicerae would seem probably to correspond with the 

 mandibles of Insects, and the antennae to be absent. In favour 

 of this view is the fact that the embryonic ganglion of the 

 mandibles of Insects is stated (cf. Lepidoptera, Hatschek, p. 340) 

 to become, like the ganglion of the chelicerae, converted into 

 part of the oesophageal commissure. 



If the above considerations are correct, the appendages of 

 the Arachnida retain in many respects a very much more prim- 

 itive condition than those of Insects. In the first place, both the 

 chelicerae and pedipalpi are much less differentiated than the 

 mandibles and first pair of maxillae with which they correspond. 

 In the second place, the first pair of ambulatory limbs must be 

 equivalent to the second pair of maxillae of Insects, which, for 

 reasons stated above, were probably originally ambulatory. It 

 seems in fact a necessary deduction from the arguments stated 

 that the ancestors of the present Insecta and Arachnida must 

 have diverged from a common stem of the Tracheata at a time 

 when the second pair of maxillae were still ambulatory in 

 function. 



With reference to the order of the development of the appendages 

 and segments, very considerable differences are noticeable in the different 

 Arachnoid types. This fact alone appears to me to be sufficient to prove 

 that the order of appearance of the appendages is often a matter of 

 embryonic convenience, without any deep morphological significance. In 

 Scorpio the segments develop successively, except perhaps the first post- 

 oral, which is developed after some of the succeeded segments have 

 been formed. In Spiders the segment of the chelicerae, and probably also 

 of the pedipalpi, appears later than the next three or four. In both these 

 types the segments arise before the appendages, but the reverse appears to 

 be the case in Chelifer. The permanent appendages, except the chelicerae, 

 appear simultaneously in Scorpions and Spiders. The second pair appears 

 long before the others in Chelifer, then the third, next the first, and finally 

 the three hindermost. 



