256 



Records of the Indian Museum. 



[Vol. XXII, 



Fig. 17. — \'ertical section through part of 

 the periostracal gland in an adult Viviparu 

 contecta (Millet) towards the end of a period of 

 growth (X Cfl. 3331. 



d., duct of gland ; g.c, gland cell ; m., fibres 

 of sphincter of the mantle; p., suffused black 

 pigment ; p.g., black pigment granule. 



The glands (fig. 14, p. 252) concerned with shell-sculpture in 

 the Viviparidae belong to two distinct series, differing in structure, 



position and function. 

 We may call them respec- 

 tively CALCIFEROUS and 

 PERIOSTRAC.\L GLANDS iu 



reference to the nature of 

 their secretions. 



The periostracal glands 

 are the smaller, less cons- 

 picuous and nearer to the 

 free edge of the two 

 series. 'J'hey lie opposite 

 the bases of the marginal 

 processes and extend both 

 upwards beneath the cal- 

 ciferous glands and down- 

 wards into the processes, 

 at the base of which they 

 open into the supramar- 

 ginal groove by a series 

 of very minute pores, one 

 for each gland (pi. iii, fig. 

 I). In the young molluscs 

 at birth each gland is a 

 simple tubule formed of a 

 single layer of gland-cells and more or less twisted in its course, 

 which is tangential to the free edge and lies amidst the thick-walled 

 cells of the interior of the marginal region. Later the glands be- 

 come contorted and the cells proliferate to form an irregular mass 

 (pi. iii, fig. 3). A definite duct is then developed, lined with very 

 minute flat epithelial cells. It makes its way to the external pore 

 from a small reservoir lying in the substance of the margin and 

 also lined with minute flattened epithelial cells. Into this the 

 secretion of the gland is evidently poured. The gland-cells (fig. 17) 

 are relatively small and ovoid in outline. Their contents do not 

 stain deeply except at birth and the}' become very inconspicuous 

 in periods of arrested growth. In those species of Viviparidae 

 that have dark-banded shells, such as V . bengalensis, V. oxytropis, 

 V. contecta, and the young of Taia intha and T. elitoralis, very 

 minute granules of black pigment are found in the cells and lining 

 the ducts of the glands, but they are absent or very scarce in 

 species with unicolorous shells, such as those of V. dissimilis and 

 L.lecyt his, exce-pt at the end of growth-periods, when dark pigment 

 may become widely suffused among the interior cells of the mantle 

 and is then by no means confined to the immediate vicinity of the 

 periostracal glands. 



The calciferous glands are larger, more numerous and more 

 conspicuous, and occupy a higher and more superficial position on 

 the mantle than the periostracal glands. They undergo, more- 



