SENSATION. . 81 



Ehizopoda; and Professor Haeckel observes, in his essay on the 

 " Origin and Development of the Sense-Organs," that " already 

 among the microscopic Protista there are some that love light, 

 and some tliat love darkness rather than light. Many seem also 

 to have smell and taste, for they select their food with great 

 care. . . ' . Here also we are met by the weighty fact that 

 sense-function is possible without sense-organs, without nerves. 

 In place of these, sensitiveness is resident in that wondrous, 

 structureless, albuminous substance which, under the name of 

 protoplasm, or organic formative material, is known as the 

 general and essential basis of all the phenomena of life." 



Again, Engelmann describes a chase of one infusorium 

 by another. The former in its free course happened to cross 

 the route of a free-swarming vorticella. There was no con- 

 tact, but it immediately gave chase, and for five seconds the 

 two darted about with the utmost activity, the chasing infu- 

 sorium maintaining a distance of about yV mm. behind the 

 chased one. Then, owing to a sudden sideward dart of the 

 vorticella, its pursuer lost the object of pursuit. The powers 

 of discrimination shown by certain deep-sea protoplasmic 

 organisms in selecting sand-grains of a particular size where- 

 with to construct their tests has already been alluded to. 



But passing now to animals in which we first meet with; 

 nerves, viz., the Medusse, it is among them also that we first 

 meet with organs of special sense. I have myself observed 

 that several species of Medusae seek tlie light, following a 

 lantern if this is moved round a bell-jar containing them 

 in a dark room. The pigmented bodies round the margin of 

 the swimming-disk were proved to be the organs of special 

 sense here concerned, and the rays in the spectrum by which 

 they are affected were shown to be confined to the luminous 

 part. It was further observed that some genera of Medusae 

 had more highly developed visual sensation than others. The 

 least efficient occurs in Tiaropsis polydiadcmata, as shown by 

 the prolonged interval of delay between the fall of a luminous 

 stimulus and the occurrence of the response. As the case is 

 an interesting one, I shall state the particulars more fully. 

 This Medusa, then, always responds to strong luminous 

 stimulation by going into a spasm or cramp ; but it will not 

 respond at all unless the light is allowed to fall upon its 

 sense-organs for a period of more than one second ; if a slip- 

 shutter is opened and closed again for a shorter period, no 



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