376 ARACHNID A. 



Summary and general conclusions. 



The embryonic forms of Scorpio and Spiders are very similar, but 

 in spite of the general similarity of Chehfer to Scorpio, the embryo 

 of the former differs far more from that of Scorpio than the latter does 

 from Spiders. This peculiarity is probably to be explained by the 

 early period at which Chelifer is hatched ; and though a more 

 thorough investigation of this interesting form is much to be desired, 

 it does not seem probable that its larva is a primitive type. 



The larvae of the Acarina with their peculiar ecdyses are to be 

 regarded as much modified larval forms. It is not however easy to 

 assign a meaning to the hexapodous stage through which they 

 generally pass. 



With reference to the segments and appendages, some interesting 

 points are brought out by the embryological study of these forms. 



The maximum number of segments is present in the Scorpion, 

 in which nineteen segments (not including the prse-oral lobes, but 

 including the telson) are developed. Of these the first twelve seg- 

 ments have traces of appendages, but the appendages of the six last of 

 these (unless the pecten is an appendage) atrophy. In Spiders there 

 are indications in the embryo of sixteen segments ; and in all the 

 Arachnida, except the Acarina, at the least four segments bear ap- 

 pendages in the embryo which are without them in the adult. The 

 morphological bearings of this fact are obvious. 



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

 chelicerse 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 prse-oral lobes, and never supplied 

 by an independent ganglion. 



The chelicerse would seem probably to correspond with the mandi- 

 bles 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. 34-0) to become, like the ganglion 

 of the chelicerse, converted into part of the oasophageal commissure. 



If the above considerations are correct, the appendages of the 

 Arachnida retain in many respects a very much more primitive con- 

 dition 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 



