64 A PRIMER OF BIOLOGY 



Thirdly, different intensities of the same stimulus 

 induce different effects in the same 'protoplasm. If 

 a number of zoospores of such a seaweed as Entero- 

 morpha (Fig. 12) be placed in a glass vessel standing 

 on a window-sill, the zoospores aggregate on the side 

 of the vessel nearest to the source of light. If a 

 strong beam of light be now thrown on that surface 

 of the vessel, the zoospores leave it and aggregate 

 on the opposite and less brightly illuminated side. 

 If the vessel be illuminated by ordinary diffuse light, 

 the zoospores distribute themselves generally in the 

 medium. The zoospores thus move towards weak 

 light, away from intense light, but are indifferent to 

 diffuse light. 



Animal protoplasm responds with mucli greater 



rapidity and more markedly to stimuli than plant 



protoplasm, and it is quite unnecessary for us even 



to cite illustrations, for they are amongst those most 



familiar to us in Nature. One point, however, we 



Sense must emphasise here, and it is that, in addition to 



organs in a g enera i sensitivity to stimuli of various kinds, \\Q 



animals. e> J JTJ- i li, 



find special sense organs developed in animals, that 



is, tissues which have become differentiated morpho- 

 logically and physiologically solely for the reception 

 of special classes of stimuli, e.g., light, contact, vapours, 

 &c. Thus we have the eye for the appreciation of the 

 form, size and colour of external objects ; the ear 

 for the appreciation of sounds ; taste-bodies for 

 distinguishing the flavours of various foodstuffs ; 

 tactile bodies in the skin and certain special structures 

 at the ends of the nerves of external or internal 

 organs for the appreciation of contact with bodies 

 likely to produce pain or pleasure ; and the nose, 

 the duty of whose sensitive inner surface it is to receive 

 impressions from volatile substances, Why are 



