ON TBE STRUCTURE OF JINLGMA ^NIGMATICA. 275 



much as the epidermal cells of many Platyhelmia are immersed 

 in the subdermal muscular and connective tissue. 



With these facts before us we have a ready explanation of 

 the hitbei'to unsolved problem of the fate of the pericardium 

 in the Anomiacea. 



In a typical Filibranch with a well-developed pericardium 

 we should find the following layers in transverse section : — 

 1. The external epithelium. 2. The outer epithelial wall of 

 the pericardial cavity. 3. The inner glandular wall of the 

 pericardial cavity adhering to the auricles or ventricle. 4. 

 The muscular wall of the auricles or ventricle. It is clear 

 that in Enigma the pericardial cavity has disappeared in 

 the vicinity of the hearty and that the external epithelium, 

 the outer and inner walls of the pericardium, and the mus- 

 cular wall of the heart itself have become more or less inti- 

 mately fused together; in particular the glandular tissue of 

 the inner pericardial wall has become incorporated with the 

 muscular wall of the heart. None the less all these forms of 

 tissue — epidermic, glandular, and cardiac muscular — can be 

 recognised by careful microscopical examination. It is 

 obvious that the disappearance of the pericardial cavity and 

 the fusion of its walls with those of the heart is iu some way 

 correlated with the torsion of the body produced by the 

 excessive development of the byssus muscle, and the attach- 

 ment of the latter to the centre of the left valve of the shell. 

 The result of this is, that the left kidney has been dragged 

 forward to pass round the byssus muscle and the left reno- 

 pericardial canal has been shifted forward far from its typical 

 position, involving a great anterior extension of the left peri- 

 cardial cavity. The corresponding structures on the right 

 side have not suffered so much displacement, but the strain 

 to which the whole system of organs must have been sub- 

 jected during the passage from the larval to the adult condi- 

 tion is sufficient to account for the obliteration of the peri- 

 cardial cavity, except in the immediate proximity of the 

 reno-pericardial canals. Sassi has suggested that the whole 

 of the venous vessel from the ventricle to the left reno- 



