CIRCULATORY AND RESPIRATORY SYSTEMS. 53 



(2) To bring blood in contact with aerating and excretory 

 organs. 



(1) No circulation is found in the simplest organisms; for 

 at different times every portion of the common tissue comes 

 in contact with fresh ingesta. But where parts are devel- 

 oped at points remote from a more or less defined alimentary 

 canal, means for the transmission of a nutritive fluid (blood) 

 from central to peripheral areas become necessary. This 

 fluid may enter the system directly or indirectly : directly, 

 through openings in the sides of the stomach into an irregu- 

 lar perigastric space, as in most Polypi, or by mouths of 

 gastric caeca (phebenterism), as in certain Arachnida ; indi- 

 rectly, by absorption through veins, as in higher Invertebrata, 

 or through both veins and lacteals, as in Vertebrata. 



Blood, thus viewed in its broadest sense, is composed of a 

 liquid (liquor sanguinis, serum) and corpuscles. The liquor 

 sanguinis very commonly is exclusively the product of the 

 walls of the blood-vessel system; but, in lowly organized 

 animals, it may receive accessions from without, as in Ento- 

 zoa (q. v.). The blood corpuscles are minute bodies, floating 

 in the liquor sanguinis. They are generally rounded, or oval ; 

 possessing with the Invertebrata an uneven or obscurely tu- 

 berculated surface, and faintly granular, and, at times, nu- 

 cleated contents. The shape and size of the corpuscles are 

 tolerably constant in the blood of the individual, but may 

 markedly vary in that of different species. With Verte- 

 brata, two kinds of corpuscles recognized: the white, which 

 corresponds to the above, and the red, which is the smaller 

 and more numerous. The red corpuscle, subject to more 

 variation than the white, may be round, disciform solid 

 bodies, as in marsipobranchiate fishes, and man ; oblong 

 and nucleated, as in JRana (frog) ; oval and of great relative 

 size, as in Proteus (salamander). Their numbers hold a rela- 

 tion to the natural heat of the animal ; being more numerous 

 in Aves than in Mammalia, and in the latter than in Pisces. 



Blood is of different colors. In Yertebrata it is red, with 

 exception of Amphioxus, where it is colorless. Commonly 

 colorless with Invertebrata, it may be greenish, reddish, or 



