1913.] Ekendranath Ghosh : Molhisca, I. 213 



the compact layer of connective tissue cells beneath the latter. As 

 they pass downwards, the fibrous strands give off branches from 

 the sides, which pass on to neighbouring strands. Just beneath 

 the glandular portion of the dermis the strands break down to 

 form an open network of connective tissue with numerous irregular 

 spaces between them. In the interspaces of the network are 

 placed numerous unstriped muscle fibres. Beneath the glandular 

 layer the fibres are arranged diagonally and longitudinally, there 

 being more fibres in the longitudinal direction. Towards the inner 

 side the diagonal arrangement becomes more prominent than the 

 longitudinal one. Lasth^, on the inner side the fibres take a trans- 

 verse (circular) course with a few longitudinal ones between them 

 In the interspaces of the connective tissue network are seen numer- 

 ous connective tissue corpuscles, some of which are very big, with 

 large nuclei. 



The blood sitms beneath the keel is elongately oval in 

 transverse section, and consists of a thick layer of muscle fibres 

 arranged transversely, lined by a single layer of flattened epithe- 

 lium. The muscle fibres are separated by thin layers of connec- 

 tive tissue and a few connective tissue corpuscles. The other two 

 pairs of blood-sinuses are broadly oval in transverse section and 

 have a similar structure. 



The inner surface of the mantle (hyponotum?), which forms the 

 outer boundary of the groove round the foot, is lined b}^ a single 

 layer of cubical cells below and of flattened cells above. The 

 nuclei are oval and are placed in a direction parallel to the surface. 

 Beneath the epithelium is a thick layer of muscle fibres arranged 

 longitudinally with little connective tissue between them. On the 

 outer side, the structure corresponds to the dermal \sLyer of the 

 mantle wall with which it becomes continuous. Just beneath the 

 muscular layer are seen numerous small pyriform unicellular 

 glands, with long sinuous ducts opening into the circumpedal 

 groove amongst the epidermal cells. The bodies of the glands 

 take a deep blue stain with hsematoxylin. The blue colour 

 becomes least aft'ected by subsequent treatment with Van Giesen's 

 stain. 



Minute structure of the foot. — The epithelium consists of a 

 single layer of narrow elongated columnar cells with more or less 

 fusiform nuclei. Between the epithelial cells are seen numerous 

 ducts of unicellular glands (similar to those opening into the 

 hyponotum) placed in the deeper layers of the wall of the foot. 

 Beneath the epithelium is a thick compact layer of muscle fibres 

 arranged longitudinally. Internal to this layer the muscle fibres 

 are arranged in loose bundles which pass vertically upwards to end 

 in the next layer. These fibres seem to be continued from the longi- 

 tudinal muscle layers. The vertical muscle fibres give off branches 

 from their sides, which pass to the neighbouring bundles. Amongst 

 these vertical fibres are seen numerous narrow longitudinal bundles, 

 which seem to be continuous with the lateral branches from the 

 vertical ones. Above, the vertical fibres change their course and 



