the Endostemite o/" Scorpio. 19 



Fairly good drawings of the endostemite in Oaleodes have 

 already been given by ^Iode.st Kittary* and by Blanchardf. 



From both of these its essential nature ean be made out. 

 It is composed of two apodemes running in from between the 

 first and second legs and meeting in the middle line under the 

 gut. Fig. 1 (PI. III.) shows the structure in situ and tig. 2 is a 

 cross section showing that the junction of the two apodemes in 

 the middle line is an interlocking apposition and not an actual 

 fusion. Its nature as a fold of the external cuticle is also 

 very a})parent from tlie section. The thickness of the chitin 

 of the fold is very irregular. Into its deeper parts (fig. 2, s) 

 staining protoplasmic matter has penetrated, and similar proto- 

 plasm ie matter binds the interlocking folds together. The 

 hindermost ends of the structure are greatly expanded for 

 the attachment of muscles. 



With this key to the understanding of the morphology of 

 the endostemite, by the kind permission of Prof. Stewart I 

 examined the fine preparation of that of Mygale at the Royal 

 College of Surgeons. The cephalothoracic segments of the 

 Araneida3, as is well known, are equally fused together, indi- 

 cation of their original separation being, however, distinct in 

 the furrows on the dorsal wall of the cephalothorax. Instead 

 of one pair of apodemes fusing in the middle line, such as is 

 found in Galeodes, we here find four pairs meeting and fusing 

 together (figs. 3, 4, and 5) |. 



This is what we should expect, owing to the larger number 

 of segments fused together. The eight apodemes further 

 radiate from a common centre, thus repeating the curious 

 radial arrangement of the segmental constrictions seen on the 

 dorsal surface. Figs. 3, 4, 5 show the arrangement of these 

 four pairs of apodemes and their place in the cephalothorax. 

 Their principal attachments to the body-wall are now the 

 points marked 1, 2, 3, 4 on the inner surface of the sternum 

 in fig. 4. In very young spiders (just hatched) the original 

 segmental constrictions running across the sternum, and now 

 reduced to these points, are still traceable as clear lines 

 running across the sternum. 



* Bulletin de la Soc. Imp. des Naturalists, vol. xxi. (Moscow, 1848). 

 t ' L'Organisatiou du Regtie Animal,' Ai'achnides, pi. xxv. fig. 9. 

 X There exist several drawings of both the dorsal and ventral aspects 

 of this structure. The earliest which 1 can hud is that of AVassmann 



by Lankester (antea, p. 18), is full of detail, but the conclusions which he 

 draws from it are vitiated by iis having mistaken the ventral fur the 

 dorsal surface. 



a* 



