482 E. A. MINCHIN. 



spread out in the cell, extending in irregular groups and rows 

 up to its extreme limits. When the cell is contracted, even 

 slightly, the nucleus lies much more internally, and is covered, 

 or even hidden, by the granules which are aggregated in the 

 centre of the cell. In the latter condition the epithelium as a 

 whole presents in surface views the appearance of compact 

 granular masses of protoplasm, each with a nucleus situated 

 more deeply, scattered about at considerable intervals one 

 from another. 



The protoplasm of the epithelial cells, apart from the granula- 

 tions already described, is clear and very difficult to make out 

 in surface views when the cell is very expanded, except just 

 round the nucleus. It appears to have a distinct alveolar or 

 reticular structure. The characteristic yellowish tint of the 

 cells is similarly most distinct when they are more or less 

 contracted and their protoplasm concentrated ; when they are 

 expanded and spread out they appear simply greyish. 



The appearance of the epithelial cells and the relative 

 quantity of their granules vary slightly in the different species 

 which I have been able to examine, though in all the general 

 characteristics are such as have been described above. 



Clathrina coriacea is a species remarkable for having 

 its dermal epithelium very granular, and the leathery appear- 

 ance from which it derives its name is due partly to this cir- 

 cumstance, partly to the very contracted condition in which it 

 is usually found when exposed at low tide, the condition in 

 which it usually figures in collections. In contorta (PJ. 41, 

 fig. 40) the granules are relatively smaller and fewer, and tend 

 to be oval or slightly elongated in form, with occasionally a 

 larger, more rounded one amongst them. The number of the 

 larger granules varies ; usually there are two or three in each 

 cell. The yellowish-brown tint is very distinct in this species. 

 In b lane a and clathrus the granules are smaller and the 

 cells appear greyish, the yellowish tint being scarcely notice- 

 able except when contracted. C. cerebrum has epithelial 

 cells rather different in appearance from those hitherto de- 

 scribed. In this form the protoplasm is remarkably vacuo- 



