Morphology of the Arachnida. 15 



e. g. Trofjulus and Stylocellas, the lc;^s are short, robust, and 

 presumably stronger ; but when the body is smaller, owing to 

 a decrease in its length, as in Phalangium^ the legs are ex- 

 tremely long and thin ; so that I think there can be very 

 little doubt that in this last-named form and its allies the 

 body is shortened and lightened that it may be raised more 

 easily upon the long stilt-like limbs. 



The structure of the Opiliones and Acari is most favourable 

 for shortening by the concrescence of the abdomen with the 

 cephalothorax, for not only is tliere no constriction between 

 tliese two regions, but the wide space that separates the coxa3 

 of the posterior walking-legs allows of the forward migration 

 of the anterior abdominal sternites beneath the lower j^art of 

 the cephalothorax. In the Pseudoscorpiones and Solifug«, 

 however, the concrescence cannot take place to the same ex- 

 tent, owing to the union in the middle line of the coxa3, this 

 union forming an impassable barrier to the forward move- 

 ment of the genital aperture. 



In the Pedipalpi, as we have seen, as also in the Arane«, 

 no fusion between the cephalothorax and abdomen is possible, 

 owing to the constriction between them. So that in Phrynus^ 

 where the body is much shorter than in Thelyphonus^ we find 

 that the shortening of the abdomen is brought about by tlie 

 reduction in length of the anterior and posterior somites. 



In connexion with tlie cephalothoracic limbs there is much 

 of interest to note. 



In the Scorpions, in which the body is very heavy, and in 

 which the large prehensorial chelfe have to be carried aloft to 

 act as clumsy organs of touch, it is not surprising that four 

 pairs of limbs are required for jnirposes of locomotion. The 

 Pseudoscorpiones also have heavy ])rehensile and tactile chela3 j 

 so, too, in this group there are four pairs of walking-legs. 

 But in the Pedipalpi, in which the body is lighter than in the 

 Scorpiones, it seems that three pairs of legs are sufficient for 

 locomotion, for the third pair of appendages fulfil the much 

 needed function of antennae. So, too, do many Spiders use 

 this same pair of legs as feelers and move with comparative 

 freedom on three pairs. The same is true of the Solifugge. 

 In both these groups, moreover, there are no heavy palpi to 

 be carried, and it must be a distinct advantage, so far as 

 agility is concerned, for the mandibles to be adapted for 

 seizing and killing prey ; for these appendages, although en- 

 larged for the purpose, must be very much lighter and more 

 easily carried than the unwieldy prehensorial palpi of the 

 Pedipalpi, Scorpiones, and Pseudoscorpiones. 



