1921.J N. Annandale & R, B. S. Sewell: Vivipara. 259 



each arranged round a small and ill-defined space. Their swollen 

 proximal extremities lie buried in the mantle, while their necks 

 extend outwards, closely pressed together, and reach the external 

 surface. 



In a vertical section they have in the mass a fibrous appearance 

 which renders them liable to be mistaken for muscle fibres unless 

 differential stains are used, and as the main axis of the cells is not 

 quite at right angles to that of the surface, sections have to be 

 slightly oblique to show their structure in detail. 



The extent to which degeneration of the calciferous glands 

 takes place in the rest-periods that succeed those of active growth 

 differ in different species and probabh' in different circumstances, 

 but they never completely disappear and even when completely 

 degenerate form a conspicuous feature of sections of the marginal 

 region of the mantle even under low powers of the microscope. 

 Generally speaking, the degeneration appears to be greater in forms 

 from a colder climate than it is in tropical species, probably be- 

 cause the alternate periods of growth and rest are more absolute 

 in the former. Major Sewell's observations on the rate of growth 

 'u Vivipara bcngalensis in Calcutta (p. 280) seem to show that 

 growth may be, if not absolutely continuous, at any rate very 

 readily revived at any time of year, whereas in Taia intha, which 

 lives at an altitude of 3000 feet in a much colder climate, few shells 

 were observed in early spring that appeared to be in a state of 

 active growth. 1 find that the European V. conteda agrees with 

 this species and its congener V . elitoralis from the same lake and 

 also with Margarya melanoides (fig. 16, p. 254), which lives at greater 

 altitudes in Western China, in having the glands very degenerate 

 in periods of rest, whereas in V. hengalensis, V. dissiini/is, V. oxy- 

 tropis and Lecylhoconcha it alters little in structure. 



In those species in which the glands become most degenerate 

 in periods of rest, as for example in Taia elitoralis, (fig. 15, p. 253), 

 the periostracal glands practically disappear, while the calciferous 

 glands are reduced to an amorphous mass in which the cell-limits 

 are distinguished with difficulty. This is most marked in their 

 "necks," which fuse together to form a structureless or almost 

 structureless layer on the external surface. When this occurs the 

 flat epithelial cells of the upper part of this surface encroach to 

 some extent on the area previously devoid of epithelium, while 

 the ciliated columnar cells of the extreme margin apparently 

 become more vigorous but do not extend upwards beyond the 

 position of the supramarginal groove, which practically disappears 

 as such. In preserved specimens in this condition I am unable to 

 detect any trace of the marginal processes and grooves, but pos- 

 sibly they may be still present in the living animal. 



The degeneration of the gland-cells is correlated with the 

 secretion of certain yellowish granules of variable size and irregular 

 shape, which are formed in them and finally become very conspic- 

 uous, even when the mantle is examined whole as a transparent 

 object under a low power of the microscope (fig. 12, p. 248). 



