OF VARIOUS UNDULOIDS 



409 



its stalk (as sometimes happens) and swims away, a new ring of 

 cilia comes into being, to encircle and support its narrow end. 



Similar unduloids may be traced in even greater variety among 

 other famihes or genera of the Infusoria. Sometimes, as in Vorticella 

 itself, the unduloid is seen in the contour of the soft semifluid 

 body of the hving animal. At other times, as in Salpingoeca, 

 Tintinnus, and many other genera, we have a membranous cup 

 containing the animal, but originally secreted by, and moulded 

 upon, its semifluid hving surface. Here we have an excellent 

 illustration of the contrast between the different ways in which 

 such a structure may be regarded and interpreted. The teleological 

 explanation is that it is developed for the sake of protection, 



127. Vaginicola. 



Fig. 128. FolUculina. 



as a domicile and shelter for the httle organism within. The 

 mechanical explanation of the physicist (seeking after the "efficient," 

 not the "final" cause) is that it owes its presence, and its actual 

 conformation, to certain chemico-physical conditions: that it was 

 inevitable, under the given conditions, that certain constituent 

 substances present in the protoplasm should be drawn by molecular 

 forces to its surface layer; that under this adsorptive process, the 

 conditions continuing favourable, the particles accumulated and 

 concentrated till they formed (with the help of the surrounding 

 medium) a pellicle or membrane, thicker or thinner as the case 

 might be; that this surface pellicPe or membrane was inevitably 

 bound, by molecular forces, to contract into a surface of the 

 least possible area which the circumstances permitted; that in 

 the present case the symmetry and "freedom" of the system 

 permitted, and ipso facto caused, this surface to be a surface of 

 revolution; and that of the few surfaces of revolution which, as 



