402 • ME. H. M. BEENAED ON THE 



the vegetative sac. On tlie other hand, they are highly specialized in the character of the 

 pedipaljis, in the crowding forward of the coxae of the limbs, in the compression of 

 the whole cephalothoracic, and in the lengthening out of the anal, segments. Internally, 

 the specialization is seen in the loss of three alimentary diverticula and of all the ostia 

 of the heart, in the cephalothorax, in the relative positions of the ganglia composing the 

 central nervous system, in the fusion of the endosternite with the highly specialized 

 diaphragm, in the loss of the waist, all consequent on the compression of the cephalo- 

 thorax above mentioned. Further, they are specialized in the forward thrust of the 

 genital aperture between the coxae of the cephalothoracic legs, in the fusions of the 

 genital glands of the two sides, and in the backward movement of the eyes along the 

 cephalic suture. 



The AranecB {especially Aviculariid<s). — These are primitive in the even character of 

 the fusion of the cephalothoracic segments, and consequent relative positions of the 

 ganglia composing the central nervous system ; in the persistence of sterna on these 

 segments ; in the leg-like pedipalj)s ; in the ocular tubercle with all the eyes grouped upon 

 it (Aviculariidae), and in the retention of filamentous limbs on the J-th and 5th abdominal 

 segments. 



On the other hand, they are highly specialized in the profound structural modifications 

 of the whole abdominal region ; in the great reduction of the heart ; in the highly- 

 complicated endosternite, and in the great backward extension of the oesophagus, so that 

 the sucking-apparatus is behind the bi'ain. 



Pseudoscojyions. — These are perhaps primitive in showing only the very earliest 

 beginnings of a waist, and in the retention of stigmatic scars along the whole length of the 

 abdomen. On the other hand, the pedipalps are as highly specialized as those of Scorpio, 

 the coxae of the limbs meet in the middle line and obliterate the sterna, and those of the 

 walking-legs are forced back by the enormous development of the coxae of the pedipalps. 

 Internally, we have the reduction of the heart, the great length of the (functional) hind- 

 gut, and the distension of all the abdominal segments into a vegetative sac. 



It will be seen that these various specializations do not lend themselves to any genea- 

 logical arrangement. They diverge entirely from one another, and cannot in any way be 

 deduced the one from the other. 



Take, for instance, one character which it appears must have early divided the 

 Arachnids into two branches, viz. the position of the movable digit of the chelicerae. 

 In one group the chelicerae remained claw-like, the two distal joints, I believe, fusing 

 to form the present movable digit ; in the other, the distal joints together made a 

 pair of pincers. All attempts at grouping, according to any siich single divergence of 

 specialization, are beset with insuperable difficvilties the moment we take other characters 

 also into accovmt. By way of example we may take the tendency of the first walking- 

 legs to become sensory in Galeodes. This tendency prepares us for the highly-developed 

 sensory charactei'S of the same limb in Thehjphomis, yet it is impossible to deduce Thely- 

 phonns from Galeodes. Their chelicerae are not formed on the same pattern. The same 

 difficulty meets one with each character in turn, and compels us to fall back upon the 

 simpler plan of deducing all separately from the hypothetical racial form. 



