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Cincinnati Society of Natural History. 



the gills, and then through the bases of the gills into the 

 auricles. The ventricle is supplied with lip-like valves, which 

 prevent the blood from returning to the auricles. The blocd 

 corpuscles are colorless, and are amoeboid in shape. 



II. Gastropoda. 



The circulatory system varies to some extent in the land 

 and fresh-water forms, but is essentially as follows : The 

 blood passes from the cavity of the foot into the opening of a 

 large vein, and from other minor veins into this vein, and 

 then passes into the branchial vein, and finally into the ven- 

 tricle, and out of this into the arteries, to be again distributed 

 over the body. There is a single ventricle and one or two 

 auricles. When the latter is the case they surround the rec- 

 tum, as in the Pelecypoda. In the Pulmonata the single 

 auricle lies always in front of the ventricle, and the blood 

 flows backward to the heart from the forepart of the animal. 

 In the Opisthobranchiata this order is reversed. In the land 

 shells the heart is situated on the left side of the animal, be- 

 tween the liver and the kidney, and below the apex of the 

 respiratory cavity, or " lung." 



In recording the pulsations of the Pelecypoda, the writer 

 extracted the animal from its shell by cutting the adductor 

 muscles near their attachment to the shell, thus separating it 

 from the animal without damaging the latter. When removed 

 the pericardium could be plainly seen, near the umbones, and 

 the heart to contract and expand. The contractions and ex- 

 pansions appeared to be wave-like, causing the cavity of the 

 pericardium to swell out considerably. In the Pulmonata the 

 heart could be plainly seen just at the periphery (sometimes 

 above it) and to the left of the aperture. The pulsations were 

 always from right to left, and the auricle seemed to push the 

 ventricle backward at every pulsation. When the shell is 

 wet the observations are easier to make, the shell being more 

 transparent in this condition. The pulsations are visibly 

 affected by heat and cold and by excitement. During the 

 hibernating season the pulse is much slower than at other 

 times — in fact it nearly ceases. 



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