CIRCULATING SYSTEM. 



203 



continue to do. Blood is scarcely known to the vulgar, 

 unless by its red colour ; and so essential is this character 

 deemed, that it appears to them little less than an abuse 

 of language to apply the term to any white or colourless 

 fluid. Even Linnaeus seems to have participated of this 

 prejudice, and to have yielded to its influence, when he 

 called the circulating fluid of the mollusca a sanies: but 

 to call it anything else than blood is apt to lead into 

 error ; for it possesses all the essential properties of blood, 

 flows in an analogous circle of vessels, and answers the 

 same purposes in the system. 



The circulating system of the mollusca consists of a 

 heart, either single, or with its parts disjoined; and of two 

 kinds of vessels, viz,, arteries and veins : and the latter are 

 supposed to perform the additional function of absorbents, 

 for nothing analogous to p . „ 



these has been yet detected. 

 The heart is very various 

 in point of figure, but is 

 always evidently muscular, 

 and has its interior strength- 

 ened with fleshy cords (co- 

 lumnae carneae), interlaced 

 in every direction (Fig. 37).* 

 It is placed in general in 

 the back, above the alimen- 

 tary canal, near to or be- 

 tween the branchiae, and in 

 a cavity usually called the 

 pericardium, and consider- 

 ed, according to Blainville 

 erroneously,-j- as the repre- 

 sentative of the same sac 

 in the vertebrate animals. 

 The arteries are very elastic, and probably muscular, al- 

 though no fibres can be detected in their gelatinous struc- 

 ture ; their coats are thicker and stronger than those of the 

 veins, which, indeed, are so extremely thin as frequently 

 scarce to be distinguished from the tissues in which they 



* Interior view of the heart of Octopus vulgaris, from Cuvier. a, The 

 aorta ; b, hranehial veins ; c, the valves ; d, columnar camepe. 



t Manuel de Malacologie, 131. " In the invertebrate animals, the heart 

 and principal artery are generally placed on the upper part of the body, 

 above the alimentary canal and largest portions of the nervous system ; 

 while in all vertebrate animals the order is reversed, the brain and spinal 

 marrow being above, the heart below, the alimentary canal." — Thomson in 

 Cyclop. Anut. and P/iy. i. 648. 



