SENSATION. 83 



object, the handle or manubrium is (in the case of many- 

 species) immediately moved over to that point, in order to 

 examine or to brush away the foreign body. This is especially 

 the case in a species which for this reason I have called 

 Tiaropsis indicans ; and here it is of interest to observe that 

 if the nerve-plexus, which is spread all over the concave sur- 

 face of the umbrella, is divided by means of an incision 

 carried in the form of a short straight line parallel to the 

 margin of the umbrella, and if a point below the line of 

 incision is touched, the manubrium is no lon.fjer able to 

 localize the seat of contact. Nevertheless it feels that con- 

 tact is taking place somewhere, for it begins actively to dodge 

 about from side to side of the umbrella, applying its ex- 

 tremity now to one point and now to another of the umbrella 

 surface, as if seeking in vain for the offending body. This of 

 course shows that the stimulus, on reaching the ends of the 

 severed nerve-fibres, spreads through the general nerve- 

 plexus, and so arriving at the manubrium by a number of 

 different routes, conveys a corresponding number of conflict- 

 ing messages to the manubrium as to the point in the 

 umbrella at which the stimulus is being applied. This irra- 

 diation of a stimulus into other nerve-fibres when the 

 stimulus reaches the cut ends of the fibres which constitute 

 the habitual route of a stimulus between two points, is ren- 

 dered the more interesting from the fact that in the case of 

 tlie external nervous plexus of the Echinodermata there is no 

 vestige of such a phenomenon. 



So much for the senses of sight (at least to the extent of 

 distinguishing light from darkness), hearing, and touch, as 

 localized in organs of special sense among the Medusae. In 

 the allied Actinite Mr. Walter Pollock and myself have ob- 5' 

 tained conclusive evidence of the sense of ^mell. For we 

 found that when a morsel of food is dropped into a pool or 

 tank containing sea-anemones in a closed state, the animals 

 quickly expand their tentacles.* It has been said that this 

 may be taken to argue a sense of taste no less than a sense of 

 smell ; but I conceive that here no distinction can be drawn 

 between these two senses, any more than we can draw such a 

 distinction in the analogous case of fish. Looking, then, to , 

 the Ccelenterata as a whole, we find that where we first meet 

 with unmistakeable organs of special sense, we also first meet 



* See Journal Linnean Society, 1882. 



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