366 MR. H. M. BERNAED ON THE 



refractive membrane (PI. XXXIV. fig. 6). Outside tlie heart ventrally tliere is a single 

 layer of nerve-fibres closely applied to it and running longitudinally (PI. XXXIV. fig. 7, n). 



Thelyphonus has nine pairs of ostia, two in tlie ceplialothorax and seven ia the abdomen, with distinct 

 traces of an extra chamber in front. 



Scorpio has seven pairs of ostia, all in the abdomen, with traces of a chamber (Newport) in the 

 cephalothorax. 



The heart of Scorpio is thus more specialized than that of either Galeodes or Thelyphonus, i. e. it has 

 lost more of its primitive segmental pairs of ostia. It is fui'ther, owing probably to the localization of 

 the respiratory organs, much more strongly developed than that of Galeodes. 



In a specimen of Euscorpio the dorso-veutral diameter of the heart was to that of the abdomen as 

 1 to 7; in a small Thelyjihonus the proportion is 1 to 9, wliereas in a specimen of Rhaw it is 1 to 21. 

 These measurements were taken between the successive pairs of ostia where the heart is a simple tube. 

 Further, the Rhax was fasting, so that this difference is not due to any great distension of the abdomen 

 on the part of the last-named. 



The ostia of the heart in both Thelyphonus and Scorpio are situated, as in that of Galeodes, on its 

 dorsal surface ; but their valves have a more special mechanism, being apparently muscular. As in 

 Galeodes, the heart is prolonged anteriorly into an aorta running towards the brain and posteriorly into 

 a vein (? or artery) which runs into the tail. 



The asj)ect of the heart, suspended by the connective-tissue strands, is very similar in the two cases. 

 In addition to the dorsal and lateral suspenders from the body-wall, tlie heart is powerfully expanded by 

 means of bundles of connective-tissue fibres, no doubt homologous with the wing-like bundles in 

 Galeodes. In the cases of Thelyphonus and Scorpio, however, these bundles are continued into muscle- 

 fibres (the veno-pericardial muscles of Lankester and Miss Beck), which run through the mass of 

 alimentary diverticula to be attached to the connective-tissue enveloping the lung-books. This is a 

 specialized mechanism to save the heart from being compressed when the abdomen is distended with food. 

 In view of the great specialization of the abdomen as a distensible vegetative sac, this mechanism 

 deserves further description. 



The heart, in both Thelyphonus and Scorpio, appears to be contained in a pericardium. But the 

 pericardium, of Scorpio at least, is really a connective-tissue membrane embracing and binding together 

 the distensible diverticula of the alimentary canal in the interest of the circulation. It runs laterally 

 down the sides of the alimentary masses and keeps the passage open between them and the outer bodv- 

 wall (PI. XXXIV. fig. 9, ?w), and originally it ran also between the segmental diverticula of the mid-gut. 

 These latter segmental infoldings have now, however, practically disappeared, leaving interesting traces 

 behind. Dorsally, they still persist as deep conical pockets, into which the wing-like expanders of the 

 heart run to be attached by means of muscle-fibres to the ventral portion of the membrane which arches 

 over a blood-space. The muscle-fibres connecting these two infoldings are probably the remains of the 

 infolding of the mcmbi'aue between the diverticula. Into these segmentally arranged blood-spaces, 

 which are the ventral i-emains of the infoldings between the diverticula, the respiratory organs project. 



The veno-perieardial muscles, as those muscles are called whicli mark the original inter-diverticular 

 blood-passages, therefore perform three functions : (1) they hold down the membrane which covers the 

 alimentary system and thus prevent that system, which, as we have seen, is liable at any moment to 

 enormous distension, from pressing on the heart ; (2) they raise the same membrane ventrally, and thus 

 prevent the alimentary system from pressing upon the lung-books and hindering the free circulation of 

 the blood along the floor of the body; and (3), being attached by means of the numerous radiating 

 fibres above mentioned to the heart itself, they may be active muscular expanders of the same. 



A pair of these veno-pericardial muscles accompanies each pair of ostia. They run down through the 

 mass of digestive diverticula, close to the dorso-vcntral muscles, whicli agrees with the suggestion that 



