20 MOLLUSC A. 



the second grating of the tubes into the large tubes of the 

 gill formed by the partitions, and up through these into 

 the long gill cavity. 



The whole of this complicated apparatus is so arranged 

 that not only is great strength and stiffness acquired, but 

 also a very large expanse of thin membrane, behind which 

 the blood passes, and which acts like a veil, l^eeping the 

 blood in and the water out, but permitting the air in the 

 water free access to the blood. 



The act of respiration among marine animals con- 

 sists simply in bringing the blood in contact with the 

 air in sea-water. This is accomplished when the blood 

 passes through the vessels of the gills, since the thin 

 membrane which surrounds all of them on the inner 

 side of the gill laminae is permeable to air. 



If part of a gill is thrown into a weak solution of caustic 

 potash, the animal matter will disappear, and leave behind 

 a number of long threads. These threads are elastic and 

 horny, and support the gills. They are secreted on either 

 side of each of the minute folds or flutings in the middle 

 layer (Figs. 7, q, 11, eli) ; and there are also similar sup- 

 ports to the vertical tubes of the inner surface of the lamina 

 at the junctions of the folds, but much broader than those 

 of the flutings. (Fig. 9, M'.) The mode of circulation 

 of the blood in the gills cannot be described until the 

 larger vessels of the body are understood. 



The heart can be exposed by lifting up and cutting 

 away or tearing off carefully the thin membrane en- 

 closing the cavity on the upper side, as in Fig. 2, h. 



The heart in a live specimen will beat regularly, and 

 the action of the different parts can be observed. 



This organ is divided into three parts, or sacs, — one 



