264 THE FORMS OF CELLS [ch. 



Morey and Draper made many years ago of melted resin are a 

 very similar if not identical phenomenon*. 



There now remain for our consideration, among the Protozoa, 

 the great oceanic group of the Radiolaria, and the little group of 

 their freshwater allies, the Heliozoa. In nearly all these forms we 

 have this specific chemical difference from the Foraminifera, that 

 when they secrete, as they generally do secrete, a hard skeleton, 

 it is composed of silica instead of lime. These organisms and the 

 various beautiful and highly complicated skeletal fabrics which 

 they develop give us many interesting illustrations of physical 

 phenomena, among which the manifestations of surface-tension 

 are very prominent. But the chief phenomena connected with 

 their skeletons we shall deal with in another place, under the head 

 of spicular concretions. 



In a simple and typical Heliozoan, such as the Sun-animalcule, 

 Actinophrys sol, we have a "drop" of protoplasm, contracted by 

 its surface tension into a spherical form. Within the heterogeneous 

 protoplasmic mass are more fluid portions, and at the surface 

 which separates these from the surrounding protoplasm a similar 

 surface tension causes them also to assume the form of spherical 

 "vacuoles," which in reality are little clear drops within the big 

 one ; unless indeed they become numerous and closely packed, in 

 which case, instead of isolated spheres or droplets they will 

 constitute a "froth," their mutual pressures and tensions giving 

 rise to regular configurations such as we shall study in the next 

 chapter. One or more of such clear spaces may be what js called 

 a "contractile vacuole": that is to say, a droplet whose surface 

 tension is in unstable equilibrium and is apt to vanish altogether, 

 so that the definite outline of the vacuole suddenly disappears "j". 

 Again, within the protoplasm are one or more nuclei, whose own 

 surface tension (at the surface between the nucleus and the 

 surrounding protoplasm), has drawn them in turn into the shape 



* See SiUi7na7i''s Journal, n, p. 179, 1820; and cf. Plateau, op. cit. ii, pp. 134, 

 461. 



t The presence or absence of the contractile vacuole or vacuoles is one of the 

 chief distinctions, in systematic zoology, between the Heliozoa and the Radiolaria. 

 As we have seen on p. 165 (footnote), it is probably no more than a physical con- 

 ^sequence of the different conditions of existence in fresh water and in salt. 



