EXAMINATION OF UNIO OR ANODONTA. 293 



lamellibranchs as live above low tide mark, and are out 

 of water for some time every day. 



c. The body cavity is now quite complicated and is 

 divided into several chambers, and contains the heart, in- 

 testine, sinus venosus, Bojanus organ, and reproductive 

 organs. 



1. The larger portion of the body cavity is now occu- 

 pied by the cavity of the pericardium (Fig. 148, v), which 

 contains the heart and intestine. 



2. The heart consists of a median ventricle (w) and two 

 lateral auricles (x) . 



(i.) The ventricle is a delicate muscular cylinder, with 

 a large cavity, upon each side of which is the aperture of 

 communication with the auricle. This aperture is guarded 

 by a pair of flaps or lips, which project inward and meet 

 in front of the opening, and thus allow the entrance of the 

 blood, but prevent its return. 



(ii.) On each side of the ventricle is a large muscular 

 auricle (x) with a small chamber, and thick spongy walls, 

 which are capable, during life, of very great distension. 



(iii.) In this section the outer ends of the auricles are 

 united to the connective tissue of the body wall ; but in a 

 section a little anterior to this their cavities will be seen to 

 communicate with the blood vessels of the gills. 



3. In the centre of the ventricle notice the cut section 

 of the intestine (p}, with its horse-shoe shaped cavity. 



4. The space between the pericardium and the roof of 

 the mantle cavity is occupied by five chambers (Fig. 

 148, , y, 2), one median and two pairs. 



In the region through which this section has passed these 

 five chambers are entirely separated from the pericardium. 



The median chamber (y) is the sinus venosus, and the 

 four others are the two non-glandular chambers (z), of the 

 organ of Bojanus, and its two glandular chambers (t). 



