CIRCULATION IN GASTEROPODA. 209 



The circulatory apparatus of the Gasteropods is less 

 complex than that of the preceding order. They have a 

 single heart, the position of which in the hody 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 the 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 vari- 

 able, but, in general, of greater capacity, and more decidedly 

 muscular. It is from one of the extremities of its great dia- 

 meter that the arterial or centrifugal system proceeds ; some- 

 times 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 sto- 

 mach, intestines, the liver, and the secretory organs of gene- 

 ration. The blood is brought back from these distant parts, 

 as in other animals, by the venous, or, as it has been happily 

 designated, the centripetal system ; the numerous branch- 

 lets 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 cha- 

 racter 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 arrange- 

 ment of their circulating system. The Tethys leporina 

 (Fig. 35), a native of the shores of the Balearic Isles, will 

 afford our first instance. In this rare and beautiful crea- 

 ture, which seems to reside constantly in deep water, the 

 heart is situated in the middle of the back, immediately 

 under the skin. Its oval and very thin auricle receives 

 the branchial veins, which trend towards it like the spokes 

 of a wheel towards the nave, and pour into it the purified 

 blood, not by one or two, but by numerous orifices. The 

 opening from the auricle to the ventricle is furnished with 

 two very distinct valves; and, as usual, the latter cavity 



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