I9I3-] Ekendranath Ghosh : Mollusca, I. 215 



the head just in front of the anterior end of the foot, and behind a 

 narrow transverse band of integument unites the bases of the 

 precephalic flaps beneath the ommatophores. The proboscis was 

 retracted in the other specimen. 



On making a longitudinal section of the everted proboscis and 

 examining the half left in connection with the pharynx, I found 

 that the pharynx, radular sac, oesophagus and two ducts of the 

 salivary glands were placed in the cavity of the proboscis. The 

 pharynx was continuous with the anterior end of the proboscis, 

 but its wall was absolutely free and separate from that of the pro- 

 boscis. The space between the pharynx and the wall of the pro- 

 boscis is filled with a mass of fragile substance, undoubtedly 

 coagulated blood. From the hinder end of the radular portion of 

 the pharynx, strands of muscle-fibres could be traced behind to a 

 narrow flattened band of muscular tissue attached to the dorsum 

 of the foot behind. 



In the other specimen, the proboscis was found to be folded 

 twice, the distal third of the organ being placed inside the invagi- 

 nated proximal two thirds. The distal third of the proboscis is thus 

 not invaginated, like the proximal two-thirds, but is left as it is in 

 the protruded proboscis. It will thus be seen that in the process 

 of inversion of the proboscis tube, the proximal end is invaginated 

 first. As the process extends from the base to the tip, the distal 

 portion {i.e. the portion towards the tip) comprising about one- 

 third the length is simply drawn in without any inversion. So 

 that as the proximal portion of the proboscis becomes inverted, 

 it gives rise to a cylindrical cavity which makes room for the 

 distal portion of the same. It has already been noted that there 

 is no connection in the way of muscular or fibrous strands between 

 this distal portion of the proboscis and the pharynx inside it. It 

 seems to be the final step in the retraction of the proboscis as, in 

 the case of being otherwise, the tip would have been drawn too 

 far behind into the cavity of the body to have an}' appreciable 

 space for it. 



Minute structure of the proboscis. — The wall of the proboscis 

 consists of the following layers : — 



(i) A single layer of flattened cells lining the outer surface of 

 the proboscis. The nuclei are elongated and rod-shaped, 

 and are placed parallel to the surface, 

 (ii) A layer of transversely arranged muscle fibres separated 

 from the epithelium by a thin layer of connective tissue, 

 (iii) A layer of longitudinally arranged muscle fibres which are 

 grouped into bundles by transversely running fibres. 

 The longitudinal bundles communicate with one another 

 by smaller bundles. Interspersed through the muscle 

 layer are unicellular glands, the contents of which take 

 a deep blue stain with haematoxylin. Their ducts open 

 into the outer surface of the proboscis, 

 (iv) On the inner side the wall contains numerous intercom- 

 municating spaces containing blood. 



