374 AKACHN1DA. 



situated near the junction of the anterior and middle thirds of its length, 

 and these are supposed to be veins* (branchio -cardiac vessels^) destined to 

 return the aerated blood from the pulmonibranchice (/) into the general 

 circulation. Whoever watches the movements of the blood in one of 

 the limbs of these creatures will perceive that, under the microscope, its 

 motion bears little resemblance to that observable in the foot of a frog, 

 or in animals possessed of an arterial and venous system completely 

 developed. So irregular, indeed, is the course of the globules, that it 

 would be difficult to conceive them to be confined in vessels at all ; the 

 whole appearance resembles rather the diffused circulation seen in the 

 larva of an insect, than that of a creature possessing vascular canals 

 arranged in definite directions. The only probable way of accounting 

 for such a phenomenon is by supposing that, in this first sketch of a 

 vascular system, if we may be pardoned the expression, the veins are 

 mere sinuses or wide cavities formed in the interstices of the muscles, 

 through which the blood slowly finds a passage. From a review of the 

 above-mentioned facts we are at liberty to deduce the following con- 

 clusions relative to the circulation of Arachnidans : The pulmoni- 

 branchice being apparently the only organs of respiration, the blood must 

 be perpetually brought to these structures from all parts of the system, 

 to receive the influences of oxygen, and again distributed through the 

 body. Such a circulation could only be accomplished in circumscribed 

 channels some destined to propel it through all parts, others to collect 

 it after its distribution and bring it to the respiratory organs, and a 

 third set to return it in a renovated condition to the heart. The circuit 

 of the blood may therefore be presumed to be completed in one or other 

 of 'the following modes : The dorsal vessel, or heart, by its contraction 

 drives the blood through numerous arterial canals to the periphery of 

 the system ; the blood so distributed gradually finds its way into capa- 

 cious sinuses, through which it flows to the branchial organs, and from 

 hence it re-enters the heart by the branchio-cardiac vessels above 

 referred to : or else the action of the heart drives a portion of the cir- 

 culating fluid into the pulmonibranchice by the same effort which supplies 

 the rest of the system, and the blood so impelled to the respiratory 

 organs becomes, after being purified, again mixed up with the contents 

 of the veins which return it to the heart. 



(965.) In the Scorpions, the circulatory system resembles that of the 

 Myriapoda, but it is more completely organized ; the heart, which, as in 

 the Scolopendra, is divided into compartments, is elongated at its posterior 

 extremity into a long caudal artery, and gives off from each chamber a 

 pair of systemic arteries, which are distributed among the viscera, and 

 also send their principal divisions to supply the muscles of the inferior 

 and lateral regions of the body, as well as the pulmonary sacs. At 

 the anterior part of the abdomen, the heart assumes the character of an 

 * Audouin, Cyclop, of Anat. and Phys., art. "ARACHNIDA." 



