252 THE FORMS OF CELLS [ch. 



becomes much more manifest when we regard it from the point 

 of view of physical and mathematical description and analysis, 

 and whose form is referable, or (to say the least of it) is very 

 largely referable, to the direct and immediate action of a particular 

 physical force. When we come to deal with the minute skeletons 

 of the Radiolaria we shall again find ourselves dealing with endless 

 modifications of form, in which it becomes still more di£6-cult to 

 discern, or to apply, the guiding principle of affiliation oi genealogy. 

 Among the more aberrant forms of Infusoria is a little species 

 known as Trichodina fediculus, a parasite on the Hydra, or fresh- 

 water polype (Fig. 8L ) This Trichodina has the form of a more or less 



flattened circular disc, with a ring 

 of cilia around both its upper and 

 lower margins. The salient ridge 

 from which these cilia spring may 

 be taken, as we have already said, 

 to play the part of a strengthening 

 "fillet." The circular base of the 

 animal is flattened, in contact with 

 the flattened surface of the Hydra 

 over which it creeps, and the oppo- 

 site, upper surface may be flattened nearly to a plane, or may at 

 other times appear slightly convex or slightly concave. The sides 

 of the little organism are contracted, forming a symmetrical 

 equatorial groove between the upper and lower discs ; and, on 

 account of the minute size of the animal and its constant 

 movements, we cannot submit the curvature of this concavity to 

 measurement, nor recognise by the eye its exact contour. But 

 it is evident that the conditions are precisely similar to those 

 described on p. 223, where we were considering the conditions 

 of stability of the catenoid. And it is further evident that, when 

 the upper disc is actually plane, the equatorial groove is strictly 

 a catenoid surface of revolution ; and when on the other hand it 

 is depressed, then the equatorial groove will tend to assume 

 the form of a nodoidal surface. 



Another curious type is the flattened spiral of Dinenympha^ 



* Leidy, Parasites of the Termites, J. Nat. Sci., Philadelphia, vm, pp. 425- 

 447, 1874—81; cf. Saville Kent's Infusoria, ii, p. 551. 



