PATELLA. 239 



secondary consequence of the special reduction of the 

 pericardium in Patella. 



The Heart. — To expose the heart of Patella, carefully 

 make a transverse incision in the front part of the roof 

 of the pericardium. Then remove this roof, first cutting 

 the fibres connecting it with the heart. The heart, thus 

 exposed, will be seen to consist of a thin-walled auricle 

 in front, and a thicker walled ventricle behind (fig. 29). 



The auricle receives blood from the pallia! gills and 

 related mantle skirt, from the roof of the nuchal cavity, 

 and, perhaps, is directly connected with the reduced left 

 kidney. Blood from the pallial gills and mantle skirt 

 is returned by the great pallial vein into the left front 

 of the auricle, while that from the nuchal cavity is con- 

 veyed by a number of small channels opening into the 

 right front part of the auricle. Perhaps among these 

 small channels, which can be very distinctly seen in a 

 fresh uninjured specimen, some bring blood from the 

 left kidney; at any rate, when the auricle is filled by 

 coloured injection the colour is communicated to this 

 kidney. 



On opening the auricle by a transverse incision, the 

 large orifice of the pallial vein can be noted, and, to the 

 right of this, a linear series of small apertures from the 

 little channels just mentioned. At the back is the 

 auriculo-ventricular septum, pierced by a transversely 

 oval opening, the edge of which is thickened. The 

 junction of auricle and ventricle is shown Tdv a well- 

 marked constriction, which corresponds to the thickened 

 margin of the auriculo-ventricular septum. 



The ventricle stretches right across the pericardium, 

 and its antero-dorsal wall is thicker than the postero- 

 ventral. As has already been said, its dorsal wall is 

 connected by fibres with the roof of the pericardium, 



