332 SCIEXCE PROGRESS. 



formed, with a small mouth aperture at its anterior end. 

 This beak persists in the Galeodidae. It is found also in 

 the Book-scorpions, in some jNIites, and, though it is here of 

 no further use as a beak, in Thelyphonus. Further, the mouth 

 parts of Scorpions, Spiders and of Phrynus can all be deduced 

 from such a beak by the suppression and modification of 

 different parts. There is thus an abundance of evidence that 

 the mouth in the primitive Arachnid was at the end of a 

 beak in all essentials like that still persisting in Galeodes. 



Thus by the aid of the Galeodidae, the initial distortion^ 

 of the primitive segmentation which transformed a simple 

 Chcetopod into the ancestral Arachnid can be unravelled, 

 while its physiological significance as an adaptation to a new 

 method of feeding is clear. We have now to show how 

 profoundly this initial adaptation modified the whole of the 

 rest of the organisation of the ancestral form and conse- 

 quently of its derivatives, the existing Arachnids. We must 

 limit ourselves to a few of the more important structural 

 features which can, with every probability, be shown to be 

 concomitant adaptations. 



T/ie ivaist or diaphragin. — This constriction, which 

 occurs, as we have seen, in all the larger Arachnids between 

 the sixth and seventh segments, must be regarded as in- 

 herited from the primitive form. Its significance is not far 

 to seek. The region anterior to the waist contains the 

 central nervous system and the powerful musculature for 

 moving the proximal joints of the limbs. Now not only 

 does the pumping action of the oesophagus suck in the 

 juices of the prey but, like a force-pump, it drives the 

 liquid food into the alimentary system, distending it to its 

 utmost capacity. This arrangement obviously requires reg- 

 ulating. Undue pressure on the nervous and muscular 

 tissues has to be avoided. Hence we find a general ten- 

 dency for the alimentary system to lose its lateral branches 

 or coeea in the anterior region, and, compared with its 

 condition in the distensible abdominal region, to be but 

 feebly developed. On the other hand, behind the waist or 

 diaphragm, the alimentary system is enormously developed, 

 and complicated systems of branching diverticula, different 



