MOLLUSCA. 



199 



Lamellibranchs breathe by 

 four plate-like gills, two on each 

 side, underneath the mantle. 

 (Fig. 98.) In the higher forms 

 the mantle is rolled up into two 

 tubes, or siphons, for the inha- 

 lation and exhalation of water. 

 The mouth opens into the stom- 

 ach, which lies imbedded in a 

 large liver, and the intestine, 



FIG. 98. Diagrammatic Transverse 

 after a few turns, passes direct- Section of Anodon, through the heart. 



. a. a. Lobes of mantle, b. b. Gills, show- 



ly through the heart. (Fig. 99.) ing transverse partitions, c. Ventricle 

 rr^i , r of heart, d. d. Auricles, e. Pericardi- 



The nervous system consists of um f Glandular sac of organ of Bo- 

 three pairs of ganglia, and the J a 5 - * Vestibu1 !; or middle sac. 



h. Venous sinus, k. Foot. A. A. Bran- 

 heart has tWO' Chambers, an aU- chial or pallial chamber. B. B. Epi- 



branchial chamber, communicating 



ricle and ventricle, and, in some with cloaca, 

 cases, two auricles and a ventricle. The ventricle pro- 

 pels the blood into the arteries, by which it is distributed 

 through the body. From the arteries it passes into the 

 veins, and is conducted to the gills, where it is aerated, 

 and is finally returned to the auricles. 



A few Lamellibranchs are fixed, as the Salt-water 

 Mussel, which hangs to the rocks by a cord of threads 

 called "byssus," and the Oyster, which habitually lies 

 on its left valve ; but the rest have a foot by which they 

 creep about. There are more than four thousand living 

 species, fresh water and marine, which range from the 

 line of shore to the depth of a thousand feet. 



The muscular impressions on the shell, (c. d., Fig. 97 ;) 

 the presence of a pallial sinus, e., which indicates the 

 possession of siphons ; the structure of the hinge, and 



