368 ME. H. M. BERNARD ON THE 



The small one-chambered heart of some Acari can be deduced, as I have shown elsewhere, from that 

 of a Spider which failed to develop the full number of abdominal segments (6). 



From a comparison of these hearts, we learn then that there were originally at least ten paii's of ostia 

 in the ancestor of the group, at least three in the cephalothorax (two persisting as functional only in 

 Galeodes and Thelyphunus), and seven in the abdomen (retained in full only in Thelyphonus and Scorpio) . 

 In the other Arachnids, varying numbers of ostia have been suppressed and the muscular heart itself 

 correspondingly shortened, although in all cases the aortic prolongation of the heart towards the central 

 nerve-mass persists. In all cases, except in Galeodes and Thelyphonus, the muscular heart is confined to 

 the abdomen. This connection between Thelyphonus and Galeodes is especially interesting. The two 

 forms are further connected, as no two other Arachnids are connected, by a transition form, Schhonotus 

 — at least in so far that the latter appears to have retained distinct cephalic lobes and free cephalothoracic 

 segments. 



The Circulation. — The various developments of the heart in the Arachnids are only 

 understood when the circulation is taken into account. Tt is only then that we can 

 appreciate the intimate connection between the circulatory, the respiratory, and the 

 alimentary systems. None of these systems can be properly studied alone. 



In Galeodes the blood, after bathing the brain and circulating in the anterior end of 

 the body and anterior appendages, has to pass back on its return from the anterior end 

 of the body towards the abdomen through an array of mviscles, large tracheal tubes, and 

 excretory tubules of the coxal glands. The proximal end of the last of these is sometimes 

 developed into a great spongy mass of tubules wiiich stretches right across the cavity of 

 the body behind and above the brain. PI. XXXIII. fig. o, eg, represents only a portion 

 of this spongy mass. Through this all the blood must filter on its retiu'n from the 

 anterior end of the body. 



It is necessary to assume that subsidiary streams are diverted into the limbs. On 

 reaching the diaj^hragm, some of the blood probably rises to the dorsal surface in front 

 of the diaphragm, to re-enter the heart by the two pairs of cephalothoracic ostia. 

 The rest passes through the neural arch (PI. XXXIII. figs. 3, 4, n), bathing the large 

 abdominal ganglion on its way, to flow backward through the great ventral sinus 

 (PL XXXIV. fig. 8, vs) which stretches through the al)domen under the alimentary 

 canal. This sinus is bordered on each side by depending diverticula. As the anterior 

 part of the abdomen is much crowded with the genital organs, tracheae, and ahmeutary 

 diverticula, the sinus is protected by a membrane {cf. PL XXXIII. fig. 4, n), which is not 

 found more posteriorly. It ceases, in fact, in the 2nd segment, and the blood escapes from 

 the sinus in all directions between the alimentary tubules, to find its way itji dorsally, and 

 posteriorly round by the stercoral jjocket to the heart. In its passage through the 

 diverticula, it is often confined here and there to special vessels of irregular shape and 

 thin membranous walls, which serve probably to guide it towards parts which would 

 otherwise be out of the regular stream {cf. PL XXXIV. fig. 8). 



The most important point which w^e notice here is the complete disorganization of the 

 regular segmental alimentary diverticula, with the accompanying complete obliteration 

 of the original inter-diverticular blood-spaces, there being no veuo-j)ericardial strands. 

 The atro^jhy of all the primary diverticula except the first, and its development into 

 a mass of branching tubules, naturally necessitated a change in the original circulation. 



