172 MR. R. I. POCOCK ON THE [June 17, 



A specialization of this nature may, I suggest, be the explanation 

 of the peculiarities of the nei-vous system of the Aranefe. 



In ' My gale,' accoixling to Blanchard, the opisthosoma is inner- 

 vated from a median strand which passes backwai-ds from the 

 prosomatic mass into the pregenital somite, the so-called pedicel, 

 and divides in the opisthosoma into a light and left cord, widely 

 separated from each other in the middle line (text-fig. 40, B, ojy.n., 

 p. 170). Each extends backwards to the spinning-appendages, 

 breaking up terminally into thieads to supply the anal region of the 

 opisthosoma. Each, moreovei', gives ofi" exteiiially thi'ee principal 

 nerves. The first and second aiise far foi'wards in the opisthosoma 

 and inneivate the genital and the following somite, with their 

 pulmonary sacs (text-fig. 40, B, lg\ Ig^"^, p. 170); the third rises 

 in the posteiior third of the opisthosoma not fai- in advance of 

 the jDoint where the tei-minal cord bieaks up into the threads above 

 desci-ibed. 



This account I have vei-ified in the case of E'phsbopus miD-inus, 

 a member of the same family as the ' Mi/gale ' dissected by 

 Blanchai-d. The median coi-d that spiings from the ])osteiioi' end 

 of the suboesophageal mass is, of coui-se, double, although the two 

 strands ai'e vei-y closely applied as they pass thiough the ' waist.' 



The exact points in the opisthosoma whei'e the three nei-ves part 

 from the piincipal strands is, in the absence of ganglionic centres, 

 difficult to ascei-tain with accux'acy, and piobably vai'ies in diflei-ent 

 types. Nevertheless the ari-angement that Blanchard has depicted 

 is in the main coi'rect. A very similai- state of things obtains 

 in the Ai'achnomoiphfe, where the opisthosoma is inneivated on 

 each side by foui' nei-ves wliich diveige fiom the common coi-d 

 that proceeds fi'om the prosoma into the genital somite of the 

 opisthosoma. 



If now, as is genei'ally assumed to be the case, the two admedian 

 nerve-sti'ands i^ejii^esent the primitive median cord, theii' wide 

 sepai'ation is not the only anomaly they present ; foi- we shall be 

 confronted with the fact that the Ai-anese ai-e the only Ai'achnida 

 known in which all the somites of tlie opisthosoma ai-e innervated 

 by cords which spring from the main trunk within the opisthosoma 

 itself. In all other orders, one (as in Liimdus) or more of the 

 somites in question receive their supply fi'om the piosomatic mass 

 with which their ganglionic centi'es have coalesced. 



I venture to suggest, then, that in the Spidei's theneives of the 

 opisthosoma i-epresent the infeiioi' system that has been desciibed 

 in the Thelyphonida?, and not the primitive median strand with 

 latei'al branches as has been hei'etofoie supposed. 



2. The Alimentary System of the Prosoma and Structure of the 



Mouth-parts in the Pedijialpi and other Arachnida. 



The fii'st point to be noticed in the alimentary system of the 



Thelyphonidffi is the modification of the sti'ucture of the coxal or 



basal segments of the chelae (appendages of the second pair). 



These segments, which were ancestrally fi'eely ai-ticulated to the 



