THE SNAIL 73 



the visceral hump has not only been coiled into a spire, but 

 has been rotated from left to right about an axis which may be 

 described as dorso-ventral, through an angle of nearly i8o°, so 

 that the anus and other organs are shifted round, and organs 

 which were primitively on the left side are actually on the 

 animal's right, and those primitively on the right side are 

 actually on the animal's left. But in the snail, as in most 

 gastropods, the primitive left organs have disappeared, and 

 those of the primitive right side are placed, as a result of the 

 rotation of the visceral mass, on the left of the original middle 

 line indicated by the rectum. 



The pulmonary chamber is triangular in outline, and lies on 

 the first turn of the spirally-coiled visceral hump. Its roof, 

 formed by the mantle-flap, is thin, semi-transparent, and richly 

 supplied with blood-vessels. Its floor is stout, muscular, and 

 convex. By contraction of the muscles the convex floor is 

 flattened, the body is protruded rather further out of the shell, 

 and the cavity of the pulmonary chamber being enlarged, air 

 rushes in through the respiratory aperture. On relaxation of 

 the muscles the body is slightly retracted within the shell, the 

 floor resumes its convex shape, the cavity of the pulmonary 

 chamber is reduced, and a part of the air is forced out. The 

 thin roof of the pulmonary chamber enables one to see the 

 organs contained in it in their natural position. The rectum 

 runs from the anus along the right upper side of the triangular 

 chamber, and passes at its apex into the second whorl of the 

 visceral mass. The excretory organ lies in the roof of the 

 chamber to the left of the rectum, and occupies nearly the 

 whole of the apical posterior part of the mantle. It consists of 

 a glandular and a non-glandular portion, the former being a 

 rather large triangular sac with folded walls lined by a glandular 

 epithelium. Its left side lies close against the pericardium, 

 and forms a deep bay to receive it. The non-glandular part of 

 the excretory organ begins at its anterior end, runs back parallel 

 to the glandular part up to the apex of the pulmonary chamber, 

 then turns sharply forward and runs close alongside and dorsal 

 to the rectum to open at the excretory pore just above and to 

 the right of the anus. The glandular part of the excretory 

 organ opens by a very small reno-pericardial canal into the 

 pericardial cavity. It is obvious that the excretory organ of 

 the snail is homologous with the organs of Bojanus of Ano- 



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