EXAMIXATIOX OF ITSIO OR ANODOXTA. 



293 



lamellibranchs as Kve 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 

 t organs. 



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

 Fpied by the ca\dty of the pericardium (Fig. 148, u), which 



contains the heart and intestine. 



2. The heart consists of a median ventricle (?c) 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 



Icommunication with the auricle. This aperture is guarded 



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



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



)lood, but prevent its return. 



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



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



rhich are capable, during life, of very gi'eat distension. 



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



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



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



■ communicate with the blood vessels of the sfills. 



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

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



4. The space between the pericardium and the roof of 

 ^the mantle ca\'ity is occupied by five chambers (Fig. 

 [148, t, y, z), 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 

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

 lorgan of Bojanus, and its two glandular chambers {t). 



