336 Proceedings of the Royal Irish Academy. 



all the trunk-segments, then not covered- by a carapace, were similar — 

 no differentiation between thorax and abdomen existing — and the- 

 head- and trunk-appendages alike. Such ancestors must have lived in 

 projCambrian times, and the segmentation of the Trilobites suggests, 

 as we have seen, that similar pre-Cambrian ancestors — with five head- 

 and fifteen trunk-segments bearing limbs — may be most reasonably 

 imagined for them. As Bernard ('94) has pointed out, the head of 

 the Cambrian Microdiscus had apparently only four segments, suggest- 

 ing that the hinder head-segments were successively absorbed from 

 the trunk. It is evident that those most generalised Crustacea must 

 have combined the primitive characters of the Leptostraca and the 

 Trilobites, and that they had the typical Arthropodan number of 

 . body-segments. A furca or a spinose telson was undoubtedly attached 

 to the anal segment. The eyes were stalked, the Leptostraca in this 

 respect being more primitive than the Trilobites. The probability 

 that the stalked eyes represent an additional post-oral pair of limbs, 

 anterior to the antennules, will be discussed later. 



Relations between the Orders of Arachnids. 



Some reference has already been made to the controversy regarding 

 the affinities of the Arachnida as a whole. As an introduction to the 

 closer study of this question, some discussion of the relationship 

 existing between the various orders that are undoubtedly referable to 

 the Arachnid class is necessary. On this subject very diverse opinions 

 have been expressed by zoologists. The question depends largely on 

 how we regard the Scorpions. 



That Scorpions are specialized Arachnids has been argued from two 

 points of view. The complete fusion of their cephalo-thoracic seg- 

 ments is compared with the presence of free or partially free segments 

 in such forms as the Palpigradi and the Solifugida. And the writers 

 who, like Lang and Bernard, believe that the lung-books of Scorpions 

 have been derived from tufted tracheae, naturally regard the order as, 

 in that respect at least, more specialized than those Arachnids that 

 breathe chiefly or wholly by means of air-tubes. 



It may help to clear the ground if, leaving the Solifugida and 

 Palpigradi aside for awhile, we compare the Scorpions with the other 

 prominent orders of living Arachnids, — with the Pedipalpi, the 

 Araneida, the Phalangida, and the Acarinida. And if that be done, 

 there seems no escape from the conclusions of Lankester ('81) and 

 Pocock ('93b), that the sequence in which these orders have just been 



