144 AQUATIC ORDERS. 



of which three intertwined shells are exhibited, the 

 animal appearing at the aperture of each. Thus formed 

 into masses, they are necessarily incapable of moving- 

 about ; and the foot, which ordinarily serves the purpose 

 of locomotion, projects beyond the head, its extremity 

 expanding into a disc furnished with a lid, closing the 

 aperture, when the animal is drawn in. The branchiae 

 form a row of fringe-like appendages, along a narrow 

 cavity. 



The seventh order of Cuvier, the Scutihranchiata^ 

 (shielded branchise,) including halyotis, fissurella, etc., 

 agrees with the preceding in the condition of the 

 aerating organs, but differs in certain anatomical points 

 of importance. 



The eighth order, Cydobranchiata^ (with branchise 

 disposed in a circular manner,) including the limpet 

 (patella) and the chiton, has the branchiae like a fringe 

 around the body, under the edge of the mantle. How- 

 ever, the general appearance and arrangement of the 

 aerating organs may differ, in the various groups — in 

 all, the blood, having undergone its necessary change, is 

 carried, by a vein, or by veins, to the auricle of the 

 heart ; it is then transferred to the ventricle, and 

 thence sent through an arterial system to circulate 

 generally. 



