RECEPTOR ORGANS 



527 



Cz 



fla 



Ot 



tension, and attachments, as to be what is known as " aperiodic " ; that is, it has no 

 definite period of vibration of its own, so that it can transmit any rate of vibration 

 indifferently. 



The perception of sound seems to have arisen somewhat late in the course of 

 evolution. There is no satisfactory evidence that invertebrates or even fishes 

 possess it. Of course, the periodic vibrations of a sounding body, if sufficiently 

 strong, can affect the touch receptors of the skin, but, as we know from our own 

 experience, the periodic impulses in the nerves from these organs do not give rise 

 to the sensation of sound ; the cerebral " analysers " necessary for the purpose are 

 not brought into play. This fact serves to confirm the view of the indifference of 

 the actual nerve impulses themselves. 



In birds and mammals the auditory organs, as valuable distance receptors, are 

 highly developed, as we 

 saw in discussing the 

 conditioned reflexes of 

 the dog. Their import- 

 ance when speech, even 

 in its most rudimentary 

 forms, makes its appear- 

 ance will be sufficiently 

 obvious. In fact, the 

 more or less musical 

 notes made by certain 

 insects, such as the 

 cricket, by the aid of 

 special apparatus, seems 

 to imply the_ presence 

 of an auditory organ of 

 some kind. 



POSITION 

 RECEPTORS 



Certain organs, pre- 

 sent in most animals, 

 even in the Medusae, 

 were supposed at one 

 time to be connected 

 with the sense of hear- 

 ing and were called 

 " otocysts." These 

 organs consist essen- 

 tially of sacs, lined with 



cells, and containing a liquid in which a loose "otolith," or several of them, is 

 freely movable. Nerve fibres terminate in the cells of the sac, and the " otoliths " 

 may be sand particles or any similar substance, insoluble in the liquid. 



Although Farre (1843) showed that these organs in the Crustacea act as 

 " delicate antennae " and have no auditory functions, it was not until comparatively 

 recently that it has been generally recognised that their function is to serve as 

 receptors for the sense of position with regard to the direction of gravity. 

 Verworn proposed that they should be called "statocysts" and the solid bodies 

 within them, "statoliths." Beer (1898) showed definitely that Crustacea have no 

 receptors for sound as such. 



Fig. 167 shows the structure of a typical "statocyst" from Pterotrachea, and 

 it is plain that the weight of the statolith will rest on different receptor cells 

 according to the position of the animal and thus afford information of its position 

 with regard to the vertical. 



An ingenious experiment of Kreidl (1893) neatly demonstrated the fact in 

 Crustacea. As is well known, these organisms periodically shed their outer 



FIG. 167. STATOCYST OF PTEROTRACHEA (A FREE SWIMMING 



MOLLUSC). 

 JV, Nerve. 



Ot, Statolith in the interior of the sac, which is filled with liquid. 

 Wz, Hair cells on the inner surface of the wall. 

 Hz and Cz, Cells with short bristles, supposed to be the sensitive cells. 



(From Claus's ' ' Elementary Text-Book of Zoology. " 

 Translated by Adam Sedgwick. London : 

 Swan Sonnenschein, 1884, p. 86.) 



