12 WOOD, 



thorax are so fused together, that it is impossible to find 

 any distinct sutures ; but in the genus Galeodes, which in 

 respect to the separateness of the head, thorax, and 

 abdomen approaches somewhat the hexapods, the thorax 

 is pretty distinctly divided into three segments. Again, 

 although the attachment of the first pair of legs in many 

 arachnids is apparently to the sternum, yet in others it is 

 very distinctly not so. Thus in the Thelyphonidae, it is 

 placed on an entirely different plane from that of the truly 

 sternal legs and the bases of the first pair of legs are 

 indeed partially covered by the base of the maxillee. In 

 the Phrynidse this is even more marked. 



For these reasons, it would seem that the anterior legs 

 of octopodous insects are really appendages of the ce- 

 phalic segments. 



In the mouth of a rapacious arachnid the most anterior 

 organs are the so-called cheliceres, which project forwards 

 or downwards from immediately under the labrum, or 

 anterior edge of the cephalothorax. Latreille was, I 

 believe, the first to regard these organs as modified 

 antennae. Siebold, who follows him in this, assigns as 

 his reasons, the cerebral origin of their nerves, and the 

 fact that they never act like the mandibles of the other 

 Arthropoda in a horizontal direction.* The first of these 

 reasons is not at all conclusive ; for if the antennae were 

 absent, it would seem, a priori, most probable that the 

 cerebral ganglion, not being called on to supply them 

 with nerve power, would send a nerve to some of the 

 mouth organs, and to which more naturally than to the 

 most anterior ? It is very doubtful whether such anatom- 

 ical facts are of any aid whatever in tracing homologies ; 

 are the arms of man any the less appendages of the occip- 

 ital vertebra, because they do not receive their nervous 

 supplies from it? Again, the cheliceres of some arachnids 

 do act in a horizontal direction, in the true. Scorpions 

 for example. The value of such a character is shown by 

 the fact, that in one order, the Pedipalpi, the Scor- 

 pions have their cheliceres acting horizontally, whilst 

 amongst the Phrynids they are vertical. Farther, the 



* Anatomy of the Invertebrates. American edition by Burnett, p. 



3^O 

 10. 



