Circulating Si/stem. " ^5. 



passes thence very often into the veins. " Were it practic- 

 able," says Dr. Fleming, " to analyse the yellow mucusi 

 which these glands contain, some light might be thrown on 

 the subject: indeed, it appears not improbable that this 

 arrangement is analogous in its functions to the urinary 

 system in the most perfect classes." {Philosophy of Zoology^ 

 vol. ii. p. 4-26.) 



The circulatory apparatus of the Gasteropoda is less com- 

 plex than that of the preceding order. They have a single 

 heart, the position of which in the body is regulated by the 

 position and symmetry of the branchiae; for, in molluscous as 

 in vertebrate animals, the heart is never far distant from the 

 aerating organs. In the greater number of the Gasteropoda 

 it is situated in the back, above the intestinal canal, at an 

 equal distance from each gill when this is paired, or obliquely 

 to tlie left, and rarely to the right, when it is single. It is 

 composed of an auricle and a ventricle: the former cavity is 

 very variable in shape, and has very thin but muscular walls ; 

 the latter is equally variable, but, in general, of greater 

 capacity, and more decidedly muscular. It is from one of 

 the extremities of its great diameter that the arterial or cen- 

 trifugal system proceeds ; sometimes by a single trunk, or 

 more commonly by two vessels. Of these, one is anterior, 

 and the other posterior : the first furnishes branches to the 

 head, to the gullet and adjacent organs ; while the second 

 sends its ramifications to the stomach, intestines, the liver, 

 and the secretory organs of generation. The blood is brought 

 back from these distant parts, as in other animals, by the 

 venous, or, as it has been happily designated, tlie centripetal 

 system ; the numerous branchlets of which, after repeated 

 inosculations, are at length united into one large trunk, which, 

 generally without the intervention of any dilatation or auricle, 

 assumes the character and office of a pulmonary artery, that 

 again divides and subdivides itself, to conduct the circulating 

 flood through all the sinuosities of the gills. 



The description just given is liable to many exceptions^, 

 were we to descend to particular families and genera; and, 

 although in a sketch of the kind I attempt to give you, it is 

 impossible to notice all their peculiarities, yet it may be useful, 

 and not void of interest, to select a few examples illustrative 

 of the most remarkable anomalies in the arrangement of their 

 circulating system. The Tethys ^eporina {^fig. 27-*), a native 

 of the shores of the Balearic Isles, will afford our first instance. 



* From Cuvier's Memoires pour servir a VHistoire et a VAnatomie des 

 Mollusques ; the most valuable work by far in this department of natural 

 history. 



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