522 ZOOLOGY. 



edge of the base of the abdomen, where it terminates. The 

 heart is generally situated above the visceral mass thus formed 

 (Fig. 183), and is composed of an aortic ventricle, and of one 

 or two auricles, destined to receive the blood from the gills, 

 In general this ventricle is fusiform, and presents a remark' 

 able peculiarity, its cavity being traversed by the rectum. 

 Finally, the nervous system consists chiefly of two pairs of 

 small ganglions, re-united by cords, but very distant from 

 each other, and placed the one above the mouth, the other 

 under the extremity of the intestine. The functions of rela- 

 tion are extremely limited, and most of these molluscs can 

 with difficulty displace themselves by pushing with the foot, 

 or rapidly shutting their shell to eject the water enclosed 

 between the valves, which gives to their body the returning 

 shock ; in general they live at the bottom of the waters, or 

 buried in the sand, and some fix themselves to rocks by 

 means of a bundle of horny or silky filaments, which spring 

 from the foot, and is called the byssus. 



612. This class is divided, according to the presence or 

 absence of lamellated branchise, into two orders. The lamelli- 

 branchiata, which comprise oysters, muscles, pearl oysters 

 (Fig. 546), the pectens (clams), the mactrse (Fig. 183), the 

 bucardia (Fig. 547), the solens or knil'e handles, the teredo, 

 &c. The brachiopoda owe their names to two kinds of fleshy 

 arms, which replace the foot ; the terebratulae (Figs 548 and 

 549) present this kind of structure. 



SUB-DlVISION 



^ 

 OF THE MOLLUSC01DES, OR TUNICATA. 



613. The animals which we re-unite here are considered 

 by most zoologists as entitled to be arranged, some amongst 

 the molluscs, others amongst the zoophytes ; but this opinion 

 seems to depend on the imperfection of the knowledge pre- 

 viously had of the structure of these beings, but now that 

 anatomy and physiology are better known, and have been 

 better studied, it may be seen that they are all formed on the 

 same general plan, and that they establish in some measure 

 the passage between the mollusca properly so called, and the 



