Sensory reactions of Holothiiria surinamensis. 291 



a secondary sense only. If Scott's (1913, p. 31) incidental obser- 

 vation that the dogfish Mustelus cams fails to react to fresh water 

 when the nostrils are plugged be substantiated, it would appear 

 that the first possibility is actually realized. Concerning the second 

 expectation, the evidence from the earthworm (Parker & Metcalf, 

 1906) and that from Holothiiria are in agreement, to the effect that 

 with the chlorides the kation effect predominates over that of the 

 anion, whereas with the human salt-taste the chlorine ion is the 

 principal stimulating agent. Stimulation by dissolved materials is 

 a process which involves certain fundamental properties of the 

 receptive mechanism as well as of the stimulant, so that such simi- 

 larities as there are between taste in vertebrates and in inverte- 

 brates may readily be accounted for. 



VII. Summary. 



(1) HoJothuria surinamensis Ludwig is reactive to tactile, vibra- 

 tory, photic and chemical stimuli. It is practically indifferent, in a 

 sensory way, to heat. 



(2) To all stimuli the order of decreasing sensitivity of the 

 parts of the body is: 



tentacles > anterior end ) posterior end > papillae , pedicels 

 (podia) > mid-body surface. 



(3) The tube -feet discs are positively stereotropic. This is 

 responsible for the righting reaction. 



(4) HoJothuria is not geotropic. The climbing of vertical walls 

 is due to the persistence of an impulse to movement. 



(5) Holothuria is photokinetic, negatively phototropic, and gives 

 a negative reaction to sudden decrease in light intensity; it does 

 not react to an increase in light intensit}^ The whole surface is 

 sensitive to photic stimuli. 



(6) These photic reactions are explainable on the basis of a 

 photochemical conception of the action of light. The fluorescent 

 integumentary pigment is possibly concerned in this matter. 



(7) Dissolved substances representing homologous stimuli for the 

 human taste qualities — sour, bitter, sweet, salt (and "alkaline") 

 are effective as stimuli. 



(8) The details of chemical stimulation are parallel to those in 



