Mollusca. 



59 



thus drawing the body away from the dorsal margin. Just under 

 the hinge a pale organ may be seen, pulsating every few seconds ; 

 this is the heart. 



12. Holding the mantle stretched, again examine the upper 

 siphonal opening ; probe to see how it extends forward above the 

 united hinder portion of the gills. In the upper part of this cavity 

 find a tube running back over the posterior adductor muscle, and 

 ending in a conical elevation ; this tube is the intestine, and the 

 opening at its end is the anus ; hence the siphon leading from this 

 cavity is called the anal siphon ; the lower siphon, which conducts 

 water to the gills and mouth, is called the branchial siphon or gill 

 siphon. Examine the gills from above, i.e. examine their dorsal 

 margins ; observe that the two outer walls of each gill are a short 

 distance apart at this edge, while below these walls unite, so that 

 if the gill be cut across, these walls, as seen at the cut, are like the 

 letter /. These diverging walls are connected by cross partitions, 

 thus forming a series of compartments within the gill, whereas if 

 these partitions were absent, each gill would be a deep, narrow, 

 undivided trough. The lateral walls of the gills are sievelike, and 

 the surface of the gill and the edges of the holes are covered with 

 cilia. The vibrations of these cilia drive the water which is around 

 the gill through these holes into the cavities within the gill ; the 

 water from each compartment of the gill passes up into the 

 chamber leading to the anal siphon. 



13. Beginning at the upper edge of the anal siphon, in the 

 middle line, cut carefully forward just above the intestine as far as 

 the umbo. This lays bare the cavity in which the heart lies, the 

 pericardial cavity. Carefully cut away the thin covering of this 

 cavity and make out the following parts of the heart : 



a. The large yellowish ventricle in the anterior part of the cav- 

 ity ; time its pulsations ; observe that the intestine runs directly 

 through the ventricle, though it has no communication with it. An 

 artery runs forward from the ventricle along the upper surface of 

 the intestine ; another artery runs from the ventricle backward 

 under the intestine. Again pull the mantle ventralward to show b. 



