RESPIRATORY CIRCULATION. 



193 



of blood vessels: so that in this case, which is that repre- 

 sented in the preceding figure (353,) the lesser circulation is 

 included as a part of the general circulation. But in all the 

 higher classes the whole of the blood is, in some part of its 

 circuit, subjected to the influence of the air; the pulmonary, 

 being then distinct from the systemic circulation. In the An- 

 nelida, for instance, the venas cav3e, which bring back the 

 blood from the system, unite to form one or more vessels, 

 which then assume the function of arteries, subdividing and 

 ramifying upon the branchial organs; after this the blood is 

 again collected by the branchial veins, which unite into one 

 trunk to form the arteries of the systemic circulation. 



Most insects, especially when arrived at the advanced stages 

 of their development, have too imperfect a circulation to ef- 

 fect the thorough aeration of the blood: and indeed a greater 

 part of that fluid is not contained within the vascular sys- 

 tem, but permeates the cavities and cellular texture of the 

 body. It will be seen, when I come to treat of respiration, 

 that the same object is accomplished by means totally inde- 

 pendent af the circulatory apparatus; namely, by a system 

 of air-tubes, distributed over every part of the body. But 

 an apparatus of this kind is not required in those Arachni- 

 da, where the circulation is vigorous, and continues during 

 the whole of life: here, then, we again meet with a pulmo- 

 nary as well as a systemic circulation, in conjunction with 

 internal cavities for the reception of air. 



In the Crustacea the circulation is conducted on the same 

 general plan as in the Annelida; the blood from every part 

 of the body being collected by the Venae Cava^, which arc 



exceedingly capacious, and 

 extend, on eacli side, along 

 the lower surface of the ab- 

 domen. They send out 

 branches, whicli distribute 

 the blood to the gills; but 

 these branches, at their ori- 

 gin, suddenly dilate, so as to 



Vol. II. 



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