14 Mr. R. I. Pocock on the 



would be preserved. But we can without much difficulty- 

 imagine that the rest of the Arachnida are the descendants of 

 our hypothetical ancestral form ; but the line of their evolu- 

 tion is quite different from that taken by the Scorpions. 



The disadvantage that this form would be under from the 

 great and useless development of its posterior abdominal 

 segments has already been pointed out^ and we have seen 

 that the difficulty has been overcome in the case of the 

 Scorpions by the conversion of these segments into a light 

 and easily carried, flexible, destructive tail. But clearlj 

 another method of dispensing with this cumbrous caudal 

 prolongation would be its suppression by the shortening of 

 tlie whole abdomen along its longitudinal axis. At first it 

 would still retain its full complement of segments, namely 

 twelve, and the last of them would be furnished with the 

 telson. But owing to the loss of flexibility in the abdomen, 

 this telson would be of no use as an organ of offence or de- 

 fence. Where it is retained, as in Thelyplionus^ Schizonotusj 

 and Kcenenitty it functions merely as an organ of touch, 

 being studded with tactile hairs, and to add to its efficacy in 

 this respect the posterior segments of the abdomen are nar- 

 rowed to form a movable supporting stalk for it. In Schizo- 

 notus it retains its original form as a single sclerite either 

 cylindrical or cordate in shape, while in Thelyphonus and 

 Kcenenia it is a long multiarticulated flagellum. In these 

 three cases greater range of movement is conferred upon this 

 instrument, and its utility is thereby increased, by the in- 

 crease in the flexibility of the abdomen brought about by 

 the constriction between it and the cephalothorax. Further- 

 more in Sch'zonotuSy in which the organ is very short, we 

 find still greater flexibility results from the secondary con- 

 striction which marks off the posterior portion of the cephalo- 

 thorax from the anterior. The result of this double constric- 

 tion is that the abdomen can be flexed right over the cephalo- 

 thorax. 



In the Pseudoscorpiones and Opiliones the telson has 

 entirely disappeared. There is consequently no great need 

 for mobility in the abdomen, and no constriction appears be- 

 tween it and the cephalothorax. These two regions of the 

 body are thus perfectly continuous throughout their width, 

 and there is nothing to prevent the two regions from fusing. 

 In the Opiliones, as we have seen, this takes place to a very 

 great extent, the result being a decrease in the length of the 

 body, which no doubt can tiiereby be carried with considerably 

 less effort by the legs. In this connexion it is interesting to 

 note that when the body is relatively large in this group, as in 



