66 ATLAS OF BIOLOGY. 



the left half of the pericardium was removed, and the auricle of that side reflected. The 

 excretory organ was then opened up from the side, and its contained excreta gently washed 

 away, x 1^. 



The fold seen embracing the genital and excretory orifices is derived from the lower lip 

 of the genital aperture. 



The mussel is at all times best dissected in its shell, but if otherwise, pins should be 

 preferably passed through the adductor muscles. If, with the animal thus quite rigid, the 

 above directions are closely followed, there can be no difficulty in following the relations of 

 the excretory organ, provided it is clearly remembered that that organ is altogether outside 

 and below the pericardium. (Compare Fig. VII., and also Figs. VI. and VII. of Plate XVI.) 



FIG. VII. Dissection of the pericardium and excretory organ from above. 



The roof of the pericardium was cut awav, the auricles in part removed, and the rectum 

 reflected. On the left side the floor of the pericardium and the roof of the excretory 

 vestibule, r&. (confluent in life), have been removed ; the point at which this is most obvious 

 indicates the interrenal aperture. 



The limits and relations of the organ of Keber are shown, and the anterior aorta has 

 been opened up on the right side.* Nat. size. 



FIG. VIII. The heart in situ, laid open from above. 



The greater part of the roof of the ventricle and of the auricle on the left side have 

 been removed, to the level of the auriculo- ventricular valve, * 3. 



The student must not be misguided by the apparent anomaly in the passage of the 

 alimentary canal through the heart ; for be it remembered that in most animals, at one 

 .period or other of their existence, the two things are closely related. 



* I cannot satisfy myself that the supposed communications between the blood- vascular system and the pericardium 

 first described by Keber (70) have any real existence. They appear to me to have been the products of undue 

 pressure used in injecting, and my own observations are entirely in harmony with those of Cattie (68) and Lankester (74), 

 and opposed to those of Griesbach (65). 



