4^'2 'Natural History of Molluscous Animals. 



Such is a very general outline of the circulation in this 

 tribe ; nor are the particular modifications to which it is 

 subject, of sufficient interest to detain us. I may just remark, 

 that the minute vessels of the branchiae form a beautiful net- 

 work, similar to that on the branchial leaves of bivalves. 



The heart has been seen pulsating in several MoUusca 

 whose bodies possess a considerable degree of transparency. 

 The pulsations are slow, and often at unequal intervals ; but 

 this irregularity may be the effect of weakness or of pain ; for 

 the animals must be placed in unnatural positions, or removed 

 from their proper element, before the observations can be 

 made ; and an attention to this circumstance may explain the 

 fact of a retrograde motion of the circulating fluid, which has 

 been observed by some naturalists. The blood itself is of a 

 bluish white colour, and glutinous consistence. Lister tells 

 us, that when he kept the blood of a snail in a vessel for some 

 days, it remained liquid and entire, not separating, in the 

 manner of human blood, into two portions of unequal densi- 

 ties ; but, when he applied heat, it readily congealed into an 

 opaque bluish coagulum, just as the human serum would 

 have done under the same circumstances. But Lister knew 

 well that the blood of these creatures was not homogeneous ; 

 for he adds, that with a good microscope it is easily shown 

 to consist of globules swimming in a limpid fluid ; that these 

 globules are truly round, and considerably exceed in size 

 those of human blood ; they are also heavier than the fluid 

 part, since they gradually sink to the bottom when kept still 

 in a glass tube. {Exercitatio Anat. de CocJileis, p. 95. Lond. 

 1694.) The late experiments of Prevost and Dumas have 

 confirmed those of the old English naturahst : they have 

 ascertained that the globules of the snail have a diameter one 

 third greater than those of man * and quadrupeds ; and, what 

 is more remarkable, they found the globules to be really 

 spherical, as Lister has asserted, although analogy would have 

 led us to a different conclusion ; for they are elliptical in birds, 

 reptiles, and fishes, to which the Mollusca are certainly much 

 more nearly allied than they are to the Mammalia. [Zoological 

 Journal^ vol. i. p. 1 78.) The globules in the bivalved Mollusca 



* The red globules of human blood, according to the observations of 

 Mr. Bauer, as corrected by Kater and others, are one five-thousandth part 

 of an inch in diameter. {Home's Comp.Anat., vol. iii. p. 4., compared with 

 p. 12.) But in the foetus, the globules, say Prevost and Dumas, differ in 

 their form and volume from those of the adult ; the former being double 

 the size of the latter (Bostock's Physiology ^ vol. ii. p. 200.), and approxi- 

 mating nearer, of course, to the size of those of Mollusca. The fact is 

 curious, when considered in relation to some speculations of Carus. 



