ON THE STRUCTURE OF ENIGMA ^NIGMATICA. 273 



and voluminous research on this snbjecfc, I may say briefly 

 that lie lias shown that in a large number of Lamellibranchs 

 belonging to different families the epithelin.l walls of the 

 pericardium are glandular and have an excretory function. 

 In many species^ and among the Filibranchia in the Arcidae, 

 Mytilidge, and Pectinidfe, the glandular tissue is localised 

 on the auricles and in some other forms, e.g. Venus 

 verrucosa (see Grobben, loc. cit., fig. 15), it extends to 

 the ventricle of the heart. The characteristic histological 

 elements of these glands are oval or somewhat irregular cells 

 with a spherical nucleus, alveolar protoplasm containing a 

 few granules, and in the latter a distinct brown concretion. 

 Though the shape, size, and appearance of these cells vary in 

 the various species examined by Grobben, they are present 

 in the pericardial glands of all, and are very distinct in 

 character from the concentric concretions found in the 

 kidney.^ The structure of the pericardial gland in Enigma 

 is shown in fig. 14. The tissues have doubtless undergone 

 contraction, and are otherwise altered by the action of spirit, 

 but it is clear enough that the bulk of the gland in the 

 vicinity of the reno-pericardial canals is formed by a mass of 

 branching cells, whose processes unite to form a reticulum. 

 Or the structure might be otherwise described as a mass of 

 vacuolated protoplasm containing numerous oval nuclei, 

 smaller, and with a denser chromatic network than the 

 nuclei of the kidney cells. This tissue may be traced along 

 the upper margin of the kidney from the reno-pericardial 

 funnel to the roots of the auricles on either side of the body, 

 and on arriving at the thickened muscular walls of the 

 auricles below the intestine it seems to thin out and dis- 

 appear. In the spaces or vacuoles of this tissue are olive- 

 brown concretions, which vary in size and appearance with 



^ Tlie concretions in the kidney are commonly said to consist of uric acid ; 

 but Lelellier (9) states tliat no uric acid is excreted by any Lamellibrancliiate, 

 and that the urinary concretions consist of calcium carbonate and acid piios- 

 phates of lime and magnesium. According to this author the Lameilibranch 

 kidney excretes uiea, the Gastropod kidney uric acid. 



