THE SENSORY RESPONSES OF CHITON 217 



These findings are in close accord with the effects of temperature 

 in producing movements among other marine invertebrates. In 

 table 2 data taken from several sources show that the temper- 

 atures inducing definite' movements indicating response are in 

 general about 15°C, and 35° to 40°C for 'cold' and 'heat/ re- 

 spectively. This is true of even Amphioxus, where Parker ('08) 

 was of the opinion that separate 'cold' and 'heat' senses are de- 

 monstrable. In Ascidia thermal sensitivity is, however, compara- 

 tively great (Hecht, '18). With the exception of Stylotella, the 

 animals concerned in table 2 were studied at Bermuda, and at 

 the same season of the year. Stylotella has been included in 

 order to show that in a sponge living at a normal temperature 

 corresponding to that of other animals with which it is com- 

 pared response to a sufficiently high temperature — identical or 

 nearly so with that inducing motor effects in Chiton, Holo- 

 thuria, Amphioxus, etc. — is clearly produced, although sensory 

 elements are not here (Parker, '10) in any way concerned. 

 The responses obtained at high temperatures do not, in our 

 belief, necessarily demonstrate the operation of differentiated 

 thermoreceptors; they result rather from the general effect 

 of high temperature upon the superficial protoplasm of the 

 animal concerned, leading to increased tactile irritability 

 (Crozier, '15 b) and other effects, and may indeed be in 

 some instances due to direct action upon muscle fibers. In 

 Amphioxus the motor behavior of the whole animal in removing 

 itself from a localized current of sea-w^ater at 39° is quick and 

 definite, but is not of different character from that to tactile 

 excitation, although evidence from experiments upon differential 

 sensory exhaustion (Parker, '08, p. 440) show that the heat- and 

 tactile-receptive mechanisms are here distinct. In Chiton, how- 

 ever, the tests so far cited do not yield conclusive evidence of the 

 presence of heat receptors. 



On the low temperature side the results are more encouraging. 

 When the animal was immersed in water at about 12°C. or slightly 

 below, the ctenidia of Chiton exhibited a definite contraction of 

 brief duration, after which they expanded. This was true even 

 .at 2°, at which temperature tactile responses were very quickly 



