378 AEACHNIDA. 



progress, pour their contents into the heart at the auricular orifices 

 upon its dorsal aspect. 



(973.) The following, then, will be the course of the circulation in 

 the Macrourous Arachnidans. The blood received by the veins from 

 the branchiae is conveyed to the heart round the sides of the segments, 

 receiving accessions from other vessels in the segments in its course, 

 and enters the heart at the posterior part of each chamber through the 

 orifices of Straus-Durckheim. The auriculo-ventricular cavity, dilated 

 by the influx of blood, begins first to contract by the action of the circular 

 fibres at the posterior part of each chamber. By this contraction, part 

 of the blood is at once propelled laterally, through the systemic arteries, 

 to the interior and sides of the body, while the remaining and chief 

 portion is forced onwards, through the valves and body of the chamber, 

 by the successive contraction of the circular fibres, into the next chamber. 

 A fresh accession of blood enters the heart at the auricular orifices in 

 the short interval of time that elapses between the contractile actions 

 of the two chambers, which interval is probably occasioned by the 

 reaction of the lateral muscular appendages of the organ. These con- 

 tractions are carried gradually onwards through the whole of the suc- 

 ceeding segments ; so that before a third chamber has contracted, the first 

 is again filled and ready to be emptied, thus occasioning, by their alter- 

 nate movements, those pulsatory motions which are so well known in 

 the dorsal vessel of Insects. The blood, propelled by these successive 

 contractions through the aorta, is distributed to the organs in the head 

 and thorax and the organs of locomotion. Part of it is also sent round 

 the aortic arches through the supra-spinal artery backwards into the ab- 

 domen, giving off its minute currents for the nourishment of the nervous 

 ganglionic cord, while another portion, intermingled with that collected 

 in the portal vessels, is sent to the branchiae. But its principal current 

 still flows in the supra-spinal artery, along the upper surface of the cord 

 to the terminal ganglion of the tail, where it divides into four streams, 

 two of which go out at the sides of the ganglion to nourish the segment, 

 while the other two, now greatly reduced in size, proceed backwards 

 along the terminal nerves of the cord, and, becoming more and more 

 subdivided in the last segment of the tail, are diffused through the sur- 

 rounding structures. These form minute anastomoses with numerous 

 small vessels, which, gradually collecting in separate trunks on the under 

 surface of the last segment, form the origin of the caudal portion of the 

 sub -spinal vessel which conveys the returning blood forwards from the 

 tail to the abdomen, to be aerated in the branchiae before it is again 

 transmitted to the heart. In like manner, the blood that has already 

 circulated through the organs of locomotion, the cephalothorax, and 

 abdomen, appears to be collected in the veins which transmit it to the 

 branchiae before it is again employed in the circulation. Throughout 

 the whole of its course along the artery in the tail, the blood is passed 



