528 COMPARATIVE ANATOMY CHAP. 



tracheae, again returning to the pericardium, and thence into the heart. This por- 

 tion of the anatomy of the circulatory system, however, requires fresh investigation. 

 The circulatory system of the Scorpionidce shows considerable similarity with that 

 of the Xiphosura, which is increased by the occurrence of an cesophageal ring and a 

 medio-ventral longitudinal vessel. It must, however, be particularly noticed that a 

 medio-ventral longitudinal vessel (subneural vessel) occurs also in many Crustacea 

 (Malacostraca), which further, in the Isopoda, is connected with the anterior end of the 

 cephalic aorta by means of an oesophageal ring. In Peripatus also a medio-ventral 

 vessel is said to have been observed. 



Araneidse (Figs. 370 B, 371). After the Scorpionidce the Araneidce possess, as 

 far as we know, the most richly developed vascular system. The heart, which runs 

 along the dorsal side of the abdomen, is enclosed in a sac-like pericardium, which is 

 itself again, as it appears, surrounded by a blood sinus. The heart has only 3 (in 

 MygaU 4 ?) pairs of openings, and is continued anteriorly into an aorta cephalica and 

 posteriorly into a short aorta or arteria posterior, and sends off laterally 3 pairs of in- 

 considerable arteries which soon open into the lacunar system of the body. The 

 arteria posterior opens into a blood sinus placed near the anus. The aorta cephalica 

 runs further forward into the cephalo-thorax, soon dividing into 2 lateral trunks, 

 which bend downwards, and after a short course break up into several arteries 

 which run to the eyes and extremities. All these arteries open into blood lacuna or 

 blood sinuses. In this definitely arranged system of lacunae and sinuses the blood 

 flows through the body in definite directions. The greater portion of it finally 

 collects on the ventral side in the anterior part of the abdomen, here takes an upward 

 direction, and thus round the book-leaf tracheae, and then again finally enters the 

 pericardium, whence it returns to the heart chiefly through the most anterior pair of 

 ostia. From the book-leaf tracheae (lungs) to the pericardium the blood flows through 

 a special vein, formed by a continuation of the pericardial wall. Since, however, the 

 pericardium itself is only a part of the ccelome, this vein also cannot be regarded 

 as a genuine blood vessel, but only as a more sharply demarcated canal-like part of 

 the ccelome, i.e. of the general lacunar system. 



In the Pseudoscorpionidse (Fig. 370, C), Phalangidse (E), Cyphophthalmidse and 

 Acarina (D) the vascular system is reduced to the heart and the aorta cephalica. 

 The heart itself, placed in the anterior part of the abdomen, becomes shorter and 

 more compact. The number of its pairs of ostia diminishes, till finally there is 

 , only 1 pair (Acarina, and Obisium among the Pseudoscorpionidce). This reduction 

 probably is caused by the anterior part of the heart losing its ostia, becoming narrow 

 and passing into the aorta cephalica, while only the posterior cardial chambers with 

 their pairs of ostia remain as a sac-like organ of propulsion. 



The heart of the Pseudoscorpionidce lies in the 3 or 4 anterior abdominal segments, 

 and in Obisium is said to have only 1 pair of ostia, in Chernes, however, 4 pairs. 

 The heart of the Phalangidce and Cyphophthalmidce has 2 pairs of ostia. Among the 

 Acarina a heart has so far been found only in the Gamasidce and in Ixodes. It is 

 probable, indeed almost certain, that many other Acarina have no heart, and in 

 general no special blood-vascular system. The same is the case in the Linguatulidce. 



In the other Arachnoidea the blood-vascular system has either not yet been in- 

 vestigated or else not sufficiently investigated for a comparative study. 



VII. The Respiratory Organs. 



The respiratory organs of the Arachnoidea are traehese, whose 1 

 to 4 pairs of outer apertures or stigmata almost always lie ventrally 



