V] OF FLUTED OR PLEATED CELLS 419 



of symmetry, such as an ellipsoid or unduloid, the folds will tend 



to be related to the ax's of symmetry, and we may expect accordingly 



to find regular longitudinal, or regular transverse wrinkling. Now 



as a matter of fact we almost invariably find in Lagena the former 



condition: that is to say, in our ellipsoid or unduloid shell, the 



puckering takes the form of the vertical fluting on a column, rather 



than that of the transverse pleating of an accordion; and further, 



there is often a tendency for such longitudinal flutings to be more 



or less locaHsed at the end of the eUipsoid, or in the region where 



the unduloid merges into its spherical base*. In the latter region 



we often meet with a regular series of short longitudinal folds, as 



in the forms denominated L. semistriata. All these various forms 



of surface can be imitated, or precisely reproduced, by the art of 



the glass-blower; and they can be seen in a contracting bubble of 



saponin, though not in the more fluid soap-bubble. They remind 



one of the ribs or flutings in the film or sheath which splashes up 



to envelop a smooth pebble dropped into a hquid, as Mr Worthington 



has so beautifully shewn. 



In Mr Worthington's experiment there appears to be something 



of the nature of a viscous drag in the surface-pellicle ; but whatever 



be the actual cause of variation of tension, it is not difiicult to 



see that there must be in general a tendency towards longitudinal 



puckering or "fluting" in the case of a thin- walled cyhndrical or. 



other elongated body, rather than a tendency towards transverse 



puckering, or "pleating." For let us suppose that some change 



takes place involving an increase of surface-tension in some small 



area of the curved wall, and leading therefore to an increase of 



pressure : that is to say let T become T + t, and P become P + p. 



Our new equation of equilibrium, then, in place of P = T/r -f- T/r', 



becomes 



T+t T+t 

 P + P = —— + —J-' 



* Certain palaeontologists (e.g. Haeusler and Spandel) have asserted that in 

 each family or genus the plain smooth-shelled forms are primitive and ancient, 

 and that the ribbed and otherwise ornamented shells make their appearance at 

 later dates in the course of advancing evolution (cf. Rhumbler, Foraminiferen 

 der Plankton-Expedition, 1911, p. 21). If this were true it would be of fundamental 

 importance: but this book of mine would not deserve to be written. 



