ON A LITTLE-KNOWN SENSE-OEGAN IN SALPA. 95 



a tactile organ. I am altogether disinclined to admit this 

 interpretation. The organ seems to me to have nothing speci- 

 fically tactile about it. The claviform appendage is wanting in 

 the most essential quality of a mediator of tactile impressions 

 — stiffness. There is nothing hard about it, it is eminently 

 soft and yielding. Compare it with the hard chitinised tactile 

 hair of an Arthropod, say the larva of a Dipteron, and this 

 difference of mechanical principle becomes conspicuous at 

 once. In the Arthropod a stiff appendage, often arranged as 

 a lever — here, the softest of papillse, a mere jelly ! 



Looking to the mere essential morphology of the organ, the 

 nature and arrangement of its constituent cells, a diflferent 

 point of view suggests itself. Fusiform sense-cells, with 

 terminal hairs, surrounded by supporting cells, and sunk in a 

 depression of the ectoderm, that is the schema of a taste-bulb. 

 And if the organ of Salpa mucronata were not surmounted 

 by the claviform appendage, or if even this appendage were 

 pierced by the smallest pore at its extremity, I suppose no one 

 would be inclined to object to its being considered a taste- 

 bulb. But the presence of the closed appendage does not seem 

 an insuperable objection to this view. The walls of the 

 appendage, though, as I have shown, they are relatively 

 thick, are after all thin structures enough, and that they must 

 be highly permeable is shown by the readiness, above noted, 

 with which the terminal hairs stain with certain reagents. 

 This reaction can be readily obtained in the living state by 

 means of gentian violet or dahlia dissolved in sea water. Thepene- 

 tration is by no means instantaneous, by no means rapid enough 

 to make the organ useful in the selection or rejection of sapid 

 food-substances (which of course, from its position, cannot be 

 its function), but it is perhaps rapid enough to allow it to be 

 useful as an indicator of the chemical quality of the water in 

 which the animal swims. 



These considerations appear to me to give much plausibility 

 to the view that the organ is either a taste-bulb, or at least 

 was one once. But there is yet another possible function 

 for which there is perhaps even more to be said — a mode of 



