I92T.] N. Annandale & R. B S. Seweii. : Vivipara. 217 



Part I.— ANATOMICAL. 

 By R. B. Seymour Seweli,. 



The bod}^ of Vivipara bengalensis, as in practically all Gastero- 

 pod molluscs, is composed of three distinct parts or regions. In 

 the fully-expanded animal the central portion forms a somewhat 

 conical-shaped foot, by means of which the animal is able to crawl 

 over the supporting surface. When fully extended the central 

 aspect of the foot or sole is, roughly speaking, a broad oval, 

 rather broader in front than behind, with a crescentic anterior 

 margin. In young examples the shape of the sole is more elongate 

 and tapers behind to a rounded point. In colour the sole is 

 slate-grey, dotted over irregularly with spots of golden yellow 

 pigment, and just within the anterior margin is a narrow but 

 distinct line of demarkation, indicated by a grey streak, which 

 corresponds to a groove between the more heavily pigmented 

 anterior fleshy border and the less pigmented muscular sole. 

 Above the expanded sole the foot rapidly tapers and on the upper 

 aspect of the posterior region is situated the horny operculum. 



The operculum is roughly oval in shape tapering somewhat 

 towards its right side,' so as to adapt it accurately to the shape of 

 the mouth of the shell. It is horny in structure and is composed 

 of a number of concentric layers so that it is considerably thicker 

 in the centre, where the opercular muscle is attached, than at the 

 margin, which is often somewhat frayed and irregular in outline. 

 The nucleus is situated excentrically about one-third of the 

 distance from the anterior margin. The colour varies in different 

 regions : around the nucleus it is a deep red-brown and immediately 

 external to this is a narrow band of a pale yellow colour : outside 

 this again the colour often deepens to a golden brown, while the 

 extreme margin is of a blackish tint. The operculum is not 

 absolute!}' flat, but is somewhat depressed in the central region 

 owing to the pull of the columellar muscle. On the body aspect 

 or lower surface the central portion of the operculum is occupied 

 by the muscular scar to which the opercular muscle is attached. 

 Surrounding this is a smooth ring, which during life is in close 

 apposition to a thin fold of glandular tissue (vide Simroth, 1896- 

 1907, pi xviii, fig. 16). In the living animal this fold almost 

 exactly| covers the whole of the body surface of the operculum 

 lying outside the scar : owing to the muscle scar being slightly, 

 asjnnmetrical, the fold is somewhat broader on the right side than 

 on the left. It is by the gland cells of this fold that the operculum 

 is secreted. 



I In the following; description the terms right and left, anterior and posterior, 

 etc., refer to the position in the fully extended condition of the aninml. 



