390 ME. H. M. BEENAED ON THE 



disappearance of the intersegmental membranes in the cephalotliorax of Arachnids is to 

 be correlated with the formation of the endoskeleton. In the Spiders they have almost 

 completely gone ; we accordingly find in this group the most complicated endoskeleton. 



Between the 6th and 7th segments the intersegmental membrane typically forms the 

 waist or diaphragm. 



In the abdominal region these membranes persist and lend the segments the great 

 distensibility which is required by the specialization of this part of the body into a- 

 vegetative sac. 



These intersegmental membranes, required for the primitive Arachnid, are not easily obtainable from' 

 a Limuloid or Eurypteroid ancestor with its hard, well-jointed, and highly specialized exoskeleton and 

 rigid head-shield. 



6. The Endosfernite. — The translocation of tlie first two pairs of limbs to positions 

 above and at the sides of the beak, and, generally, tbe great muscular development of the 

 first six segments, have led to deep infoldings of the intersegmental membranes to form 

 different kinds of endoskeletal structures, according to the different specializations and 

 degrees of compression of these segments. We have the endosternite and the Avaist or 

 diaphragm. 



The nearest equivalents in other Arthropods to these endoskeletal structures are the 

 infoldings separating the bead and thorax, and the thorax and abdomen, of the Hexapoda,^ 

 and the endopbragmal system of Astacus and its nearest relatives. These are all 

 structures formed in the same method — viz., by the longitudinal compression of segments 

 leading to an infolding of intersegmental membranes, in each case in response to some 

 special method and area of compression. 



The endoskeleton of Limulus, which has been compared with that of the Arachnids, has had an 

 entirely different origin, and is therefore not an homologous structure. It receives its full elucidation in 

 A2)us as the fusion of the intersegmental tendinous tracts of the ventral longitudinal muscles from which 

 the contractile tissue has atropliied. This origin is in as perfect harmony with the specialization of 

 the primitive segmentation of the Merostomata as is the rise of the Arachnidau endoskeleton with the 

 specialization of the Arachnida. 



7. The Waist or Diaph'cujm. — The two chief regions of the body are separated by a 

 waist or diaphragm, the origin of which can perhaps be traced primarily to the folding 

 together of the appendages of the 7tb segment towards the median line over the genital 

 aperture to function in various ways as sexual organs. It is not improbable that the 

 slight constriction caused by this arrangement was taken advantage of to cut off the 

 hinder region as a vegetative sac, with a narrow neck into which the fluid food could be 

 pumped in great qiiantities without fear of interfering, by undue distension of the 

 alimentary canal, with the mechanism of locomotion and prehension of the anterior (6) 

 seo-ments. From this j)oint of view, it is probable that the diaphragm, which is most 

 perfect io Scorpio, implies the former presence of a waist. The primitive waist, how- 

 ever, need not have been so pronounced as that of the Spiders and Pedipali^i, which is 

 almost certainly a specialization of this arrangement. The Galeodidie seem to combine 

 the two, forming both a slight waist and a fairly complete diaphragm. On the one 

 side, then, we have the constriction completed to form a true waist ; on the other, the 



