1 894.] 



THE BRITISH NATURALIST. 



229 



Let us now examine some of the lowest forms of animal life, and 

 observe how far we may suppose that they are sensitive to these 

 various forms of stimulation, and how far they are provided with 

 special organs for their appreciation. The common Amoeba is a 

 small mass of nucleated protoplasm devoid of any special sense organs. 

 Any part of this organism is capable of performing the functions of 

 digestion, of respiration, of contraction, and of reproduction ; and we 

 must conclude that every part is as sensitive as any other part to 

 the changes in the surroundings which would, by a higher animal, be 

 appreciated by its sense organs. While watching it under the field of 

 the microscope, we realize that it is throughout sensitive to touch, for 

 any part of it which comes in contact with a foreign body either 

 contracts, and draws away from it, or gradually closes round it and 

 ingests it. If we warm the stage we find that its movements become 

 more active, and we conclude that it is sensitive to heat. 

 Whether it has the rudiments of any sense of hearing is less easily 

 determined ; but it will exhibit some sensitiveness to vibrations in the 

 surrounding water, of which sound vibrations are only a special class. 

 That it has a rudimentary sense of taste is seen by observing that it 

 is differently affected by organic and inorganic bodies with which it 

 comes in contact, more frequently ingesting the former than the latter. 

 Smell, as already stated, being only, in aquatic animals, a more 

 delicate form of taste, a power of recognizing a smaller quantity of a 

 substance of a certain chemical nature, we must conclude that if Amoeba 

 has a rudimentary sense of taste it has also a rudimentary sense of 

 smell. I am not aware that Amoeba behaves in any special way in the 

 presence of light, but on the analogy of the action of light on other low 

 forms of life, we may assume that light does affect it ; probably, at least, 

 its vital processes are more active in light than in darkness. Here, 

 then, we have a speck of protoplasm which, although it has no 

 special sense organs or nervous system, shows evidence of reacting 

 in a crude way to most of the forms of stimulation to which we 

 react in virtue of our elaborate sense organs and nervous 

 systems. 



If we next examine some of ciliated Protozoa, not so simple as 

 Amoeba, but still unicellular, we can observe a distinct specialization of 

 certain parts for sensory purposes, and also a specialization of other 

 parts for motion. In the first place we note, what might also have 

 been noted in some of the forms of Amoeba, that the external 

 layer of the body is distinctly differentiated from the inside, and 

 so must be the part which is affected time after time by changes in 

 the surroundings. 



In Vorticella we are still dealing with a unicellular organism, but we 

 can recognize that special functions are assigned to special parts of the 



