ARACHNIDA. 



necessary for diffusing the blood through the system ; but in the 

 Pulmonary Arachnidans numerous vascular trunks are given off 

 from both sides of the dorsal heart, and are dispersed in all direc- 

 tions. All the branches proceeding from the sides of the dorsal 

 vessel are presumed to be of an arterial character, with the excep- 

 tion of a few large canals 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 pulmo-branchite 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 dif- 

 fused circulation seen in the larva of an insect, than that of a crea- 

 ture 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 

 conclusions relative to the circulation of Arachnidans : The 

 pulmo-branchite 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 distribu- 

 tion, 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 con- 

 traction drives the blood through numerous arterial canals to the 

 periphery of the system : the blood so distributed gradually finds 

 its way into capacious 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 



* Dr. Audouin, Cyclop, of Anat. and Phys. art. AIIACHNIDA. 



