338 ME. H. M. BEENAED ON THE 



at no oreat distance from eacli other ; ventrally, they are inserted in the median line, 

 Those descending between the stigmata draw up the fold between the opercula, and 

 thus, by bringing the inner edges of these latter closer together in the median line, help 

 to close the stigmata (in PI. XXX. fig. 16 these muscles are attached to the ridge m. 

 but anteriorly to the plane of the section). This medio-ventral insertion of the dorso- 

 ventral muscles has been already Correlated with the approximation of the coxae in the 



middle line. 



The last two pairs of dorso-ventral muscles are in contact for a short distance in the 

 middle of their course (PL XXX. figs. 9, 14). 



The chief function of these muscles can hardly fail to be associated with the contents 

 of the abdomen. They serve to compress and hold it together, especially when greatly 

 distended by large quantities of liquid food pumped into the alimentary diverticula. 

 Their relations to the abdominal diverticula will be referred to in the section on the 

 alimentary canal. 



Taking the cephalotlioracic with the abdominal dorso-ventral muscles, w^e have in 

 Galeodes traces of a series from the constriction between the 3rd and 4th segments to 

 that between the 6th and 7th abdominal segments, in all ten pairs. 



These dorso-ventral muscles are a marked feature in the abdomens of Arachnids. They are apparently 

 least developed in the Spiders, in which animals the abdominal sac is very highly specialized. Wasmanu 

 (74 a) figures four pairs in the abdomen of Mygale. Schimkevitch (62) describes three pairs for Epeira. 



In Scorpio there are in the abdomen seven pairs of powerful muscles attached either wholly or chiefly 

 to the flat surfaces of the plate-like tergites and steniites. Dorsally they are, as in Galeodes, attached 

 on each side of the heart ; while ventrally, instead of meeting in the middle line, they diverge widely so 

 as to be attached on each side at the inner edges of the limb-areas (PI. XXVII. fig. 16, la). The first 

 of these dorso-ventral muscles in the abdomen of Scorpio, corresponding with the pair which in Galeodes 

 run down to the fold between the genital opercula (PI. XXX. fig. 9), have become attached to the 

 endosternite through the diaphragm ; this has been brought about by the forward thrust of the genital 

 aperture between the cosje of the legs, right under the endosternite. 



In the Pseudoscorpions there are also seven pairs of dorso-ventral muscles in the abdomen attached, 

 as in Scorpio, to the flat surfaces of the sternites and tergites. In these cases, judging from Galeodes, 

 each pair of such muscles originally belonged to the intersegmental constriction next behind it. 



In Phrynus there appear also to be seven pairs of dorso-ventral muscles, while in Thehjphonus there 

 are apparently eio-ht pairs. In both cases, as well as in Scorpio, the muscles are attached ventrally in 

 two rows with a row of sterna between them, close to the inner edges of the limb-areas [cf. PI. XXVII. 

 fio-s. 16 17, 18, /a). lu Tlielyphonus there are therefore in all twelve pairs of dorso-ventral muscles, 

 four in the cephalothorax and eight in the abdomen. 



We thus have, in all, traces of twelve pairs (or thirteen, if those in the cephalothorax of the Pseudo- 

 scorpions are rightly honiologized) of these very primitive muscles in the Arachnids, Thehjphonus having 

 retained the Teatest number. This is important; it shows that Thelyphonus, with its tail of three 

 seo-ments, is less specialized than Scorpio, with its tail of five segments. In the ancestral form the 8th 

 abdominal segment was not specialized into a tail-segment. 



The connection between these dorso-ventral muscles and the alimentary diverticula will be discussed 

 in the section dealing with the latter system. 



The "Feiw-pericardial" Iluscles. — These, which form a series characteristic of the 

 Arachnids, are entirely absent in Galeodes. They will be described in the section on the 

 respiration, and the reason of their absence in Galeodes wall be discussed. 



