of the Anatomy of the Squalus Maximus. 237 



angles to that of the artery, and then another set of muscles 

 is employed to force it into the artery. 



This mechanism, common to most fishes, and so different 

 from what is met with in land animals, where the blood is 

 forced by the action of the left ventricle into the different 

 parts of the body, appears to be intended to prevent too great 

 a force being employed, at any one time, in impelling the blood 

 through the gills. 



Having traced in these different gradations the guards upon 

 the circulation through the gills in fishes, I am led to extend 

 my remarks to the hearts of a lower class of animals. In the 

 Mollusca there is no reason for putting guards upon the action 

 of the heart, as in fishes, because in them the blood first sup- 

 plies the different parts of the body, and only in its return 

 passes through the gills ; but there is a regulation of another 

 kind, by means of which the circulation is increased or dimi- 

 nished, according to the activity or torpor of the animal. In 

 the teredines, where a bonng engine, requiring a great mus- 

 cular power to work it, is almost constantly employed, the 

 heart consists of two auricles and two ventricles ; the auricles 

 strong, cylindrical, and having valves between them and the 

 ventricles; the ventricles themselves very strong, so that in 

 fact there is a heart, composed of two auricles and two ven- 

 tricles, both acting at the same time, and the blood is hurried 

 on by a double power to supply the muscles of the boring 

 shells, and, in this part of its course, has a bright red colour. 

 In the Oyster, on the other hand, the heart consists of only 

 one auricle and one ventricle, both very weak in their muscular 

 power, when compared with those of the teredines, although- 

 belonging to an animal of larger dimensions ; but the heart is 



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