Cxliv THE VOYAGE OF H.M.S. CHALLENGER. 



partly by direct observation of tlie reaction of the living organism towards various 

 stimuli, and partly by the comparison of the different conditions of existence under which 

 Eadiolarians exist, both in the most various dejDths of the ocean and in all climatic zones 

 (see note A). In general the Radiolaria seem to be sensitive to the following stimuli ; 

 (1) pressure (see note B) ; (2) temperature (see note C) ; (3) light (see note D) ; 

 (4) chemical composition of the sea- water (see note E). The reaction towards these 

 stimuli, corresponding to the sensation of pleasure or dislike which they call forth, is 

 shown in various forms of motion of the protoplasm, changes in the currents in it, 

 ■contraction of the central capsule, changes in the size, position, and form of the pseudo- 

 podia, changes in the volume of the calymma (by the evacuation of water), &c. Among 

 the sensory functions of the Eadiolaria must be especially mentioned their remarkably 

 develoj^ed perception of hydrostatic equilibrium (see note F), as well as their perception 

 of distances, so clearly shown in the production of equal lattice-meshes and other regularly 

 formed skeletal structures (see note G). 



A. I can add but little to the communication wliich I made twenty-four years ago regarding 

 sensation in the Eadiolaria (L. N. 16, pp. 128-131). The most important point would be the great 

 difference in irritability which must obtain between the pelagic, zonarial and abyssal Eadiolaria, 

 which may be assumed from a consideration of their very different conditions of existence as re- 

 gards pressure, Light, warmth, nutrition, &c. It is natural to suppose that the numerous abyssal 

 Eadiolaria, discovered by the Challenger, which live at great depths (2000 to 4500 fathoms) in 

 complete darkness, in icy cold and under an enormous pressure, must have quite different sensations 

 of pleasure from their pelagic relatives wliich live at the surface of the sea under an equatorial sun. 

 Karl Brandt has recently added much to our knowledge regarding the special action of different 

 vital conditions upon the various Polycyttaria and the degrees of their irritability (L. N. 52, 

 pp. 113-132). 



B. Eegarding the sensation of pressure or sensation of touch of the Eadiolaria and the various 

 ■degrees of their mechanical irritability, see L. N. 16, p. 129; L. N. 41, p. 4G4. 



C. Eegarding the sensation of warmth or temperature-sense and its dependence upon different 

 climatic relations, see L. N. 16, p. 129; L. K 52, pp. 114-129. 



D. Eegarding the sensation of light, compare L. K 16, p. 128; L. N. 42, p. 304; L. K 52, 

 pp. 102-104, 114. 



E. Eegarding the sense of taste of tlie Eadiolaria or their peculiar sensitiveness towards the 

 different chemical composition of the water, change in its salinity, presence of organic impurities, 

 &c., see L. K 16, p. 130; L. N. 52, pp. 103, 113. This chemical irritability seems to be the most 

 highly developed sense in the Eadiolaria, even more so than their mechanical irritability. 



R The perception of hydrostatic equilibrium among the Eadiolaria is immediately visible from 

 the position which their bodies, floating freely in the water, assume spontaneously, and from the 

 symmetrical development of the skeleton, which by its gravitation necessitates a definite position. 

 It may be assumed that the development of the various geometrical ground forms which con-espond 

 to a definite position of equilibrium, is the result of this particular kind of perception (compare 

 §§ 40-45). 



