122 Coiqding of Abdomen with Cephalothorax in Ricinulei 



the third segment, so that, even when viewed from above, the most prominent portion of 

 the lateral corners of the abdomen (which are turned a little inwards) is formed b^^ the 

 third sternite. 



On the ventral surface of the ahdumen (PI. IX., tig. 1 g) we find an adaptation of the 

 sternite of the third segment, serving the same purpose but quite differently contrived. The 

 anterior border of the plate in question (v^) is rather deeply emargiuate on either side 

 between the lateial corners of the abdomen (which as we have said are formed bj' that 

 plate) and the middle, which does not reach so far forward as the corners. Behind this emargi- 

 natiou the ventral surface of the sternite exhibits on either side a somewhat deep concave 

 portion, of which the posterior margin is uniformly curved from a point situated well forward 

 on the projecting portion in the middle and near the middle line, to a point on the side 

 situated a long distance behind the fore-end of the lateral corner, and somewhat nearer the 

 middle line than the latter. The concave portions are thus bounded anteriorly and posteriorly 

 by somewhat curved lines, but laterally by straight lines which converge a little towards the 

 posterior end of the body. The bottoms of the concave portions are not quite plane, and at 

 the same time somewhat deeper behind and distally, where the walls of the excavation a 

 little overhang the excavation, which consequently in these places has the shape of a shallow 

 pocket; this is not visible on our figure. Into these excavated spaces the posterior free 

 portions of the fourth pair of coxaj fit together with their keels. Onh' a portion of the 

 dorsal surface of the fourth pair of coxas is attached to the ventral surface of the cephalo- 

 thorax : the distal portion, about two-fifths of the length of the coxse, is free, and exhibits 

 along its posterior border a free expansion, which is continued as a horizontal keel (PI. VIII., 

 fig. 2 g, k) with convex margin. The dorsal surface of this keel forms a continuation of 

 the dorsal surface of the free part of the coxa, whilst the ventral surface of the keel is 

 sharply marked off from the ventral surface of the coxa. The whole of the free part of 

 each coxa of the fourth pair fits into the concave space of the ventral surface of the .sternite 

 of the third abdominal segment, the keels more particularly fitting into the pocket-like hollow 

 places above described at the posterior extremity of the concave spaces. The consequence is 

 that when the abdomen and the cephalothorax are coupled together no more of the third 

 sternite is seen than the triangular portion between the two excavations (PI. VII., fig. 1 h). 



To complete the description of this part of the abdomen we may add that those portions 

 of the sternite of the third segment and of the tergite of the second segment, which lie 

 between the soft part of the peduncle and the projecting lateral corners, are strongly 

 chitinized, and form on either side two somewhat concave surfaces which turn forward, for 

 which reason they are seen on PL IX., figs. 1 /" and 1 g, very much foreshortened, as deeply 

 shaded places ; they are separated by a horizontal keel, of which the top edge is represented 

 (k) in both the figures as the anterior margin of the shaded parts just mentioned. 



When the coupling is effected the projecting lateral comer of the abdomen enters into 

 the space between the free distal part of the fourth coxa on either side and the lateral 

 portion of the dorsal shield of the cephalothorax, and fits into a hollow bounded by the lateral 

 portion of cephalothorax, and — beneath — by the upper surface of the free distal portion of 

 the third coxa. 



We have described this remarkable mechanism principally according to its appearance in 

 Cryptosteinma Karscliii. It is found in all the species we have seen, but the shape of the 

 component parts, at any rate of the concave spaces on the third sternite, is not quite the 



