312 ME. H. M. BERNAED ON THE 



The obliteration of the sternal plates by approximation of the basal joints of the limbs along the 

 ventral middle line, which is thus a characteristic of the Galeodidfe, is found more or less marked in all 

 Ai'achnids, at least for certain segments. The causes of it are not easy to see. 



Figs. 15-18 (PI. XXVII.) and 1, 2 (PI. XXVIII.) show, in series, the ventral surfaces of Galeodes, 

 Scorpio, Thelyphonm, Phrynus, a Spider and a Chernetid, the true sternal areas being in all cases 

 shaded. In Phrynus [Taratitula tessellata, fig. 18) sternites are found along the whole length of the 

 body, except in segments 7 and 8, where they are covered by the large genital operculum. In Thelyphonus 

 (17) the sternites of the 1st and 2nd segments forming the labium ai'C now enclosed between the coxse of 

 the pedipalps. The sternum of the third segment is a fold beneath the solid plate which represents the 

 sternite of segment four, having been displaced by the pedipalp. A very small plate is all that remains 

 of the sternite of segment five. A large triangular plate is sternite six. In Scorpio (fig. 16), the first 

 and second sternites are no longer visible externally. The short pointed labium found in sections is all 

 that now remains of them. The sternites of segments three and four have quite disappeared. Sterna 

 five and six are present as a triangular plate in some genera. In others, even this plate has been almost 

 obliterated secondarily, not by the approximation of the coxse, but by the forward movement of the 

 genital aperture and opercula. The sternite of the 7th segment is covered by the opercula, while 

 the sternites of the eighth and following four segments are clear. 



With regard to the abdominal sternites in Scorpio, Thelyphunus, and Phrynus, I believe the lateral 

 boundary of the sternite to be the attachment of the dorso-ventral muscles ; outside these are the areas 

 which I refer to vanished limbs [cf. PI. XXVII. figs. 16, 17, 18 Za). 



In the Pseudoscorpions (PI. XXVIII. fig. 2, ? sp.) the sternites have vanished from the cephalothorax, 

 persisting perhaps only in the labium, and in a triangular plate between the last pair of legs which is 

 visible in some genera. On the abdomen, the conditions appear to resemble those in Galeodes; we 

 have the sternal plates, at least, very frequently, in pairs divided by a median line continuous with that 

 separating the coxse of the cephalothorax. On the other hand, the dorso-ventral muscles do not always 

 meet in the ventral middle line, but their points of attachment may have shifted. 



In the Araneae, the sterna of the cephalothoracic segments are fused to form a solid plate, but in young 

 Epeirids just hatched (when cleared) this plate is found marked as in the figure (PI. XXVIII. fig. 1) 

 We here see that sterna 1 and 2 are fused to form the labium, while sterna 3, 4, 5, 6 are well marked. 

 The condition of the abdomen in the Spiders is not quite clear. From the approximation of the 

 spinning-glands in the median line, I should be inclined to think that the sternites in this group, as in 

 Galeodes, had disappeared from the abdominal segments. 



In the Phalangidse, the sterna of the cephalothorax have disappeared, either by the approximation of 

 the coxse, or by the forward thrust of the genital aperture. In the Acari all conditions are found : 

 the sterna, in some genera, seem to be well developed on all the segments except perhaps the 1st and 

 2nd. I have, however, made no study of the group. 



We thus have, in the Arachnida, almost every possible variation in this matter. The sterna have 

 almost completely vanished from the cephalothorax of some Scorpions, of the Phalangidse, and of the 

 Pseudoscorpions. In the Spiders they have been apj)arently obliterated from the abdomen. In the 

 Pedipalpi, traces of them persist along the whole body, having completely vanished only in the anterior 

 end of the abdomen, where they are covered over by the large genital operculum*. Lastly, in 

 the Galcodidic the sternites have vanished along the whole length of the body, the only persistent traces 

 being the supporting plate of the labium and the triangular piece between the last pair- of legs. 



These diff"erent departures from the primitive conditions of the segments are so very distinct that it is 

 impossible to deduce the existing Arachnids one from another : they can only be different speciali- 

 zations of some common racial form in which the sternites were distinct along the whole length of the 



* Laurie (47) describes a plate roofing the genital vestibule to which a pair of dorso-ventral muscles are attached ; 

 this is almost certainly the sternite of the 2nd abdominal segment. 



