the Endostermte oj Scorpio. 21 



The endostcrnite of Scorjno is somewliat complicated by 

 the strong longitudinal compression of the thorax having 

 caused it to fuse posteriorly with the " diaphragm " (figs. 

 8 and 11, d). This " diaphragm," like that of Galeodes, is the 

 homologue of the great constriction between the sixth and 

 seventh segments forming the " waists " of other Arachnids. 

 In both Galeodes and Scorpio it is pierced by the dorsal vessel 

 {h), the alimentary canal (a), and the nerve-cords (n), and in 

 Galeodes by traeheaj (t). In Galeodes it lies some distance 

 behind the posterior edge of the endostcrnite, there being no 

 longitudinal compression of the thorax, and it also slopes 

 from above backwards {cf. figs. 1 and 10). In Scorpio the 

 secondary longitudinal compression of the cephalotliorax, 

 which is more marked than in other Arachnids, es])ecially on 

 the ventral surface, has caused the " diaphragm " to slope 

 from above forward {cf. figs. 8 and 11). In fig. 8 the poste- 

 rior edge given to the (shaded) endostcrnite is naturally 

 arbitrary, as, except where it passes under the alimentary 

 canal, it is fused with the " diaphragm." There seem to be 

 four points of fusion. The ridges (r), caused by the dorso- 

 ventral muscles behind the " diaphragm," have fused with 

 the postero-lateral tendons {t) of the endostcrnite, to which 

 the muscles themselves are now secondarily attached ; and, 

 further, the horizontal plate of the endostcrnite itself is 

 attached by two processes (e) to the anterior face of the 

 " diaphragm " which runs slantingly under it. Fig. 11 illus- 

 trates the relative positions of the endostcrnite and the 

 " diaphragm ; " it shows only the fusion of the plate of the 

 endostcrnite with the latter, and is not intended to represent 

 a true horizontal section. Fig. 9 is a transverse section 

 (through the line % in fig. 11) of Euscorpio showing the 

 endostcrnite with a portion of the " diaphragm " (Lan- 

 kester's " subneural portion ") which has been thrust forward 

 under it by the ventral median portion of the seventh segment. 

 With this portion of the '' diaphragm " [d] so thrust forward, 

 the extreme posterior ends of the endostcrnite (e) fuse on each 

 side of the nerve-cord. 



The figures which have hitherto been given of the endo- 

 stcrnite of Scorpio really represent, then, a composite body, 

 viz. the endostcrnite and the " diaphragm " fused together. 

 This point is of great importance, as it seems to me to 

 establish beyond question the complete distinctness between 

 the endosternites of Limidus and Scorpio. In order to 

 maintain the contrary it would be necessary to show that the 

 endostcrnite of Limidus was once part of a " diaphragm," a 

 " diaphragm " or waist being typical of Arachnids. 



