96 ARTHUR BOLLES LEE. 



action according to which, as it seems to me, this mechanism 

 cannot help acting. The cellulose or " tunicin " mantle of 

 Salpa is a fairly tough but highly watery and highly hygro- 

 metric jelly. Given a papilla such as this claviform appendage, 

 with sensory hairs inserted into its tip, if the density of the 

 ambient water be changed, the shape and dimensions of the 

 papilla must be changed in consequence, it must elongate or 

 shorten, and the hairs must be stretched or slackened. If the 

 animal pass into less dense sea water, that is, a medium richer 

 in pure HgO, water will be taken up, the appendage will swell 

 and elongate, and the hairs will be pulled. If the animal be 

 carried into water richer in salts the claviform appendage will 

 shrink, and the hairs must be relaxed. 



And it seems to me that I have evidence that these actions 

 do actually take place. I have noted above that the shape of 

 the appendage is variable. I have to add here that its surface 

 is typically smooth, but very frequently corrugated or wrinkled 

 into lines of shrinkage. This is especially the case with 

 specimens that have been kept for some time in captivity, and 

 with such as have been treated with chloral hydrate or solutions 

 of intra- vitam staining agents. Even a little methylen blue 

 added to the sea water, in so small a proportion as not perceptibly 

 to stain the hairs, may produce this effect. And in such 

 specimens the hairs themselves, which in perfectly fresh 

 specimens have a smooth and regular contour, are seen to have 

 a markedly corrugated outline, and sometimes a very great 

 degree of curl or twist near the tip. 



And, all things considered, I incline to regard the organ as a 

 sensory areometer or hydrometric apparatus. 



