Morphology of the Arachnida. 13 



It now remains to be seen it" any beneficial results can be 

 ascribed to the structural modifications that wc have endea- 

 voured to trace riglit through the Arachnida, starting with 

 the Scorpioncs and ending with the Acari. 



At the outset of this article it was concluded upon mor- 

 phological and enibryological grounds that the ancestor of 

 the Arachnida was an animal composed of 18 distinct somites, 

 which were divisible into tliree categories, composed of 6 

 somites each. The appendages of the anterior six were of 

 large size and were set apart as organs for locomotion and 

 for prehension and mastication of prey; those of the succeed- 

 ing six were of small size, the posterior four of them disap- 

 pearing in connexion with the development of four pairs of 

 abdominal breathing-organs. The six somites of the last 

 category were without appendages, and constituted what may 

 be termed a caudal termination to the body. Now it seems 

 perfectly clear that an Arthropod of this description would be 

 most unfitted for terrestrial life on account of the clumsiness 

 of its build. In fact, being forced to drag along a long, 

 trailing, heavy, legless abdomen it could not be otherwise 

 than sluggish, and would consequently find no little difficulty 

 in gaining a livelihood by the capture of other terrestrial 

 Arthropods, which, if not otherwise protected, are usually 

 characterized by extreme activity. It would consequently be 

 an undoubted benefit to our hypothetical ancestor if the 

 caudal termination to its body could be either dispensed with 

 or turned to some account. 



The latter end could be without difficulty attained by the 

 lateral compression of the segments, which would confer con- 

 siderable flexibility upon them. Moreover, since, as we have 

 seen, the last segment was furnished with a post-anal, pro- 

 bably pointed, sclerite, it seems clear that a formidable 

 weapon of attack and defence might be thus constituted. But 

 its greatest use would probably be to put a speedy end to the 

 struggles of prey that had been seized by either one or the 

 other of the prehensorial limbs. In this capacity its efficiency 

 would be greatly increased by the development of a poison- 

 gland in the telson. A concomitant advantage in the deve- 

 lopment of this " tail " would be a loss in the weight to be 

 dragged by the limbs owing to the decrease in the size of the 

 segments. 



In some such manner as this we may imagine that the 

 group of Scorpiones has been evolved. But it does not seem 

 probable that any other group of Arachnida has been derived 

 from them. For it appears hardly likely that any variations 

 tending to the obliteration of so useful an organ as tlie " tail" 



