2/6 A. S. PEARSE. 



cal opening was near the surface of the water, or came out of 

 the sand and climbed up the side of the jar in which they were 

 kept, or increased the size of the body and ceased to respond to 

 shadows and other sHght stimuH, and in some cases even cast 

 out the visceral organs. 



4. Change in Density of Medium. — Tliyone can stand a marked 

 increase or decrease in the density of the water in which it lives 

 without serious interference with its activities. In order to test 

 the effect of increased density an individual was placed in a one- 

 liter beaker which had sand in the bottom to a depth of six cen- 

 timeters. This beaker was filled with sea water and allowed to 

 remain on the table in the laboratory from July 12 until August 

 7 (twenty-five days), when the animal died. During this time the 

 water had evaporated so that the specific gravity had increased 

 from 1.024 to 1.052. This animal showed no signs of the changes 

 due to unfavorable environment until July i, when it came out 

 of the sand where it had been previously buried and lay on top. 

 From this time it began to show signs of degeneracy, the skin was 

 blistered off in spots and the body took on a peculiar elongated 

 form. Many of the reactions continued to be normal however 

 and the usual shadow reaction (which is described in the next 

 section, p. 277) was easily induced the day before it died. On the 

 morning of August 7 the animal failed to respond to shadows 

 but gave the withdrawing response when it was poked gently. 

 Four hours later it was dead. Subsequent examination showed 

 that the visceral organs were still in place and had not been cast 

 out on account of the increased density of the water. 



The effects of a decrease in density were next investigated. In- 

 dividuals were placed for a time in various mixtures of sea and 

 fresh water. They were then returned to sea water and their 

 subsequent condition noted. The results of these experiments 

 are shown in Table II. No individual ever attempted to burrow 

 while it was in a solution of lesser density than sea water, although 

 sand was always placed in the bottom of the dish. The body 

 remained contracted, spouting was infrequent and the tube-feet 

 were never attached and seldom even extended over much of the 

 body surface. In the cases where the animals survived their im- 

 mersion in the mixtures they soon began regular spouting move- 



