ELEMENTARY VITAL PHENOMENA 229 



d. Movements ~by Change of the Specific G-ravity of the Cell 



Among the wonderful forms of animals, mostly of glassy trans- 

 parency, that lead a pelagic life in the upper strata of the sea and 

 lately as plankton have become the object of detailed investi- 

 gation, there are many that are endowed with the remarkable 

 capacity of slowly rising or sinking in the water without the use 

 of any locomotor organs. These are especially the Radiolaria, 

 Ctenophora, and Siphonophora. Some unicellular, fresh-water 

 organisms, such as Actinosphceriwn, also possess this power. Since 

 all external causes for this mysterious suspension, such as currents 

 of water, may be excluded, and since the movement of special 

 organs of the body does not share in it, it can depend only upon 

 changes in specific gravity, and this has been demonstrated. As 

 has already been seen, 1 protoplasm is heavier than water. Hence 

 a cell that lies upon the bottom can raise itself only when 

 substances that are lighter than water appear and accumulate 

 in the protoplasm. 



It is well known that certain fresh-water Rhizopoda, especially 

 Arcella and Difflugia, which are provided with delicate capsules, 

 are heavier than water, and usually creep about upon the bottoms 

 of ponds and puddles between particles of mud and decaying 

 leaves, can actively raise themselves by developing a bubble of 

 carbonic acid in their protoplasmic bodies ; when it has become 

 sufficiently large, they rise to the surface like a small balloon. 

 Engelmann ('69) first carefully investigated this fact. At 

 times in a culture-vessel containing Difflugia, when conditions 

 favour the development of carbonic acid in the protoplasm, 

 the movement of individuals from the bottom to the surface 

 becomes epidemic. If the carbonic acid is then given off, 

 the individuals sink again to the bottom. In this manner 

 there may arise in nature a very considerable change of habita- 

 tion, which under certain circumstances, as when the Protista 

 have come under unfavourable conditions, can be of great useful- 

 ness to the species. 



In an analogous manner take place the rising and sinking of the 

 Radiolaria and, in all probability, those of the Ctenophora and 

 many other pelagic animals. Thalassicolla nucleata, e.g., is a large 

 globular radiolarian of 3-4 mm. in size, which represents a single 

 cell, the nucleus of which, surrounded by protoplasm, lies in a 

 spherical central capsule (Fig. 92). The whole extracapsular 

 protoplasm is filled with innumerable vacuoles, so that it appears 

 like a mass of foam, and it is bordered externally on the side of 

 the sea- water by a solid layer of jelly. This vacuole-layer is the 

 portion of the cell that is lighter than the sea-water, and 



1 Cf. p 97. 



