GASTEROPODA. 



Fig. 190. 



391 



the exception of the Scutibranchiate and some 

 of the Cyclobranchiate orders, the heart is 

 single, consisting of an auricular and ventri- 

 cular cavity, and is interposed between the 

 branchial or pulmonary vessels and the system, 

 receiving the aerated blood from the respiratory 

 organs, and propelling it through the body. 

 The heart of Aplysia (Jig. 191, e, g) or of the 

 Snail, (Jig. 190, <>, p) will exemplify its ordi- 

 nary structure. The auricle varies slightly in 

 shape in different genera, but is always ex- 

 tremely thin and pellucid, containing in its 

 coats muscular bands of great delicacy. The 

 ventricle is provided with stronger walls, and 

 is generally separated from the auricle by a 

 valve, formed of two pieces. The heart is en- 

 closed in a pericardium, but its position is re- 

 gulated by that of the branchiae; and from the 

 great diversity of arrangement which we have 

 found the latter to present, a corresponding 

 want of uniformity in the locality which the 

 heart occupies may be readily expected. \\ e 

 shall select two forms of the respiratory organ 

 as examples of the variable position of the 

 heart, and as illustrations of the usual distribu- 

 tion of the bloodvessels, viz. the Snail, (vide 

 CIRCULATION, //if. 322, and the Doris, fg. 

 321,) and afterwards notice the principal aber- 

 rations from the ordinary disposition. In the 

 Snail, the blood derived from the whole body 

 is brought by great veins, performing the func- 



tions both of the vena cava and of a pulmonary 

 artery, to the plexus of vessels lining the floor 

 of the respiratory cavity ; after here undergoing 

 the needful aeration, it enters the heart, from 

 whence it is driven into the aorta. The aorta 

 immediately divides into two trunks, one dis- 

 tributed to the liver, the intestine, and the 

 ovary; the other supplying the stomach, the 

 oral apparatus, the organs of generation, and 

 the foot. In the Slug the arteries are perfectly 

 white and opaque, and their ramifications, 

 which may be traced with great readiness, are 

 extremely beautiful. 



In Don's (Jig. 321) the heart is, in conse- 

 quence of the position of the branchiae around 

 the anus, removed quite to the posterior extre- 

 mity of the body. The blood derived from all 

 parts of the body is conducted by large veins 

 to the respiratory organs ; the pulmonary arte- 

 ries which return it from thence unite into a 

 circular vessel (A, i), surrounding the anus, 

 and from this arise two vessels, emptying them- 

 selves into the auricle. The aorta, on issuing 

 from the heart, divides into two large vessels, 

 the first supplying the intestinal canal, stomach, 

 and duodenum, the organs of generation, the 

 foot, and the mouth ; whilst the other lugt 

 trunk is entirely distributed to the liver. 



In Tritunia the heart is placed near the 

 centre of the body, and the auiicle itself resem- 

 bles a cylindrical vessel placed transversely 



