478 ZOOLOGY. 



versed 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 relation 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 branchiae, into two orders. The lamelli- 

 braiichiata, which comprise oysters, muscles, pearl ovsters ( Fig. 

 460), the pectens, the mactre (Fig. 160), the boccardefl (Fig. 

 461), the solens or knife handles, the teredo, &c. The l>ra- 

 chiopoda owe their names to two kinds of fleshy arms, which 

 replace the foot ; the terebratula? (Figs. 462 and 4(33) pre- 

 *sent this kind of structure. 



SuB-DlTISIOX 

 OF THE MOLLUSCOIDES, OR TUXICATA. 



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 

 zoophytes. They all have a distinct digestive tube, turned en 

 itself, and open at both extremities, and have a very well deve- 

 loped branchial apparatus (Fig. 465); most of them also pre- 

 sent vestiges of a nervous system, but have no ganglionary 

 ring like the mollusca, properly so called; finally, alnmst ail 

 multiply by granulations as well as by ova, and thus form 

 aggregation* of individuals more or less completely confounded 

 with each other. 

 These animals are all aquatic, and are formed on two prin- 



