OBSERVATIONS OF THVONE BRIAREUS. 28 1 



during the winter. I had therefore expected to see the burrow- 

 ing reaction take place as the temperature was reduced but all 

 three of the individuals remained perfectly quiet as the ice formed 

 around theiu and the posterior end was not withdrawn. 



From the experiments described it is evident that Tliyoiie is 

 comparatively insensitive to thermic changes and that it is able 

 to react through a wide range of temperatures. The maximum 

 and minimum vital limits are in the neighborhood of 40° C. and 

 0° C. respectively. 



VIII. Experiments to Determine whether the Integrity 

 OF the Nervous System is Essential to Reactions. 



The classical work of Romanes ('85) showed that a single fifth 

 of the body wall of a sea-urchin was able to carry on locomotion 

 without any of the visceral organs. Such fragments executed 

 righting reactions and showed the same positive phototropism 

 which was characteristic of entire animals. Mead (:oi) kept 

 detached starfish arms aliv^e for as much as three months and 

 they retained their powers of locomotion and gave the usual 

 righting reaction. Von Uexkull ('97) found that pieces of sea- 

 urchin would react to mechanical stimulation but the responses 

 to shadows depended on keeping the system of radial nerves in- 

 tact. These and other observations show that the nervous system 

 of the asteroids and echinoids is little centralized in some respects, 

 though Jennings (:o7) has recently described some remarkable 

 instances of association in the starfish. Clark ('99) found that 

 cutting the oral nerve ring made no appreciable difference in the 

 reactions of Synapta. Henri (:o3, -.Oia, :oib) showed that 

 nerve centers exist in the radial nerve trunks of Stichopus regalis 

 and that reflex muscle contractions could be induced throuc^h 

 them by stimulating the skin. He states that such radial nerve 

 centers control only a limited portion of the body musculature 

 and that reflexes which involve more than one of the longitudinal 

 muscles must pass through the oral nerve ring. 



As has been stated, the nervous system of holothurians con- 

 sists of a circum-oral ring which gives rise to five radial nerve 

 trunks and these are connected through their finer branches which 

 anastomose to some extent. This system has been modified some- 



