370 MR. H. M. BERNARD ON THE 



required to flow laterally through the specialized luugs. The blood for the digestive tubules is received 

 direct fi-om the heart. On each contraction of the heart the blood is forced, through several pairs of 

 latero-ventral openings (Lankester, "arteries" in Anclroctonus) , among the alimentary tubules. At 

 these vents the heart is in contact with the " pericardium," so that the blood is discharged under the 

 latter and can only find its way back to the heart after circulating (1) through the mass of digestive 

 tubules, and (2) (PI. XXXIV. fig. 9, cf. arrows on the right) through the upper laminae of the lung-books, 

 whence it escapes into the lateral blood-passages, which convey the blood, close under the skin, to 

 the "pericardium." Among the digestive tubules the blood circulates freely, with, however, membranous 

 vessels here and there (as in Galeodes) for the purpose of guiding it to all parts. 



This cannot be considered as a primitive arrangement, inasmuch as the breaking up of the primary 

 simple alimentary diverticula into the branching tubules must have itself been a secondary modification. 



These specializations have clearly necessitated the formation of the continuous membrane round the 

 ■whole alimentary system, which keeps the complicated streamings apart. It is this membrane which, 

 ■where it passes under the heart, functions as a pericardium. 



In the Pseudoscorpions we have an entirely different specialization ; the heart, as in all Arachnids, 

 propels the blood on to the brain, and after flowing among the tissues, coxal glands, spinning-glands, &e., 

 in the cej^halothorax, finds its way back along the floor of the body to the abdomen. On its way it is 

 apparently divided into two latero-ventral streams by the development of a large median ventral diver- 

 ticulum of the mid-gut. Each of these streams has to pass through a felt-work of extremely fine 

 tracheal tubules (27 and 10), which have developed at the tips of the original simple tracheal invagination 

 of the 2nd and 3rd abdominal segments. The blood is thus aerated, as it flows into the abdomen, by 

 two pairs of specialized trachese. 



Reasons have, however, already been given for believing that these posterior trachea persisted until 

 comparatively recently, inasmuch as we still have the seven pairs of functional inter-diverticular blood- 

 passages persisting along the dorso-ventral muscles (PL XXXIV. flg. 3). 



The blood flowing ventrally and laterally along the abdomen finds its way up, partly through the 

 lateral muscle or inter-diverticular blood-passages, and partly through the coils of the hind-gut, above 

 the genital glands, into a dorsal median sinus, from which it is drawn by the fan-shaped posterior end of 

 the heart {cf. p. 367). 



The circulation in the Aranese has been cai'efully studied and described by Claparede (25), and it 

 agrees with the scheme common to all Arachnids which we have so far described. The blood discharged 

 forward by the heart flows back freely through the lacunse of the cephalothorax ; passing through the 

 waist (there being no neural arch, as in those Arachnids which possess diaphragms), it flows on each side 

 of the alimentary canal (PL XXXIII. fig. 6). On reaching the abdomen a great part of it flows 

 immediately to right and left through the specialized lung-books of the 2nd segment (which have 

 been pushed forwards). The rest appears to run on, in the Tetrapneumones, to flow up through a 

 second pair of lungs, or, in the Dipneumones, freely through the alimentary diverticula towards the 

 pericardium. According to Claparede, the posterior prolongation of the heart is an artery, as may 

 perhaps be the case in Scorpio. In both cases I should consider it as a secondary specialization, in the 

 interest of the spinning-glands or tail. It may be noted that the ostia in the hearts of both the Spiders 

 and Scorpio would apparently admit of blood flowing either forward or backward. In Galeodes it can 

 flow only one way, i. e. forward ((/. PL XXXIV. fig. 6). 



The Blood-corpuscles. — The blood-corpuscles are, as a rule, round cells with large 

 nuclei. They measure from 4- to 10 n. I have been unable to determine their origin, 

 as no blood-forming gland, nor any j)rocesses of division of the corj)uscles, could be 

 detected in my preparations. 



In Scorpio the blood-corpuscles are very uniform in size and shape, ca. 8-12 fi. As one source of 

 these free cells, I have elsewhere (11) suggested the detached mid-gut cells, which find their way into the 



