No. 569] 



REGENERATION 



290 



just slightly in excess of five, the pentameric number, 

 and the average number when two Polian vesicles are 

 present is 10.263. This ratio is only partly maintained 

 when three vesicles are present, for the average number 

 is then 12.400, and in the two cases with four vesicles the 

 average was just twice the pentameric number. It is 

 therefore evident from the facts shown in this table that 

 with an increase in the number of Polian vesicles there 

 is associated a strong tendency for the retractor muscles 

 to take on a split character. If it were not for the fact 

 that the split character shows considerable variation in 

 the same individual one might suggest that the tendency 

 to divide is correlated with the greater functional activ- 

 ity of the water vascular system as evidenced by the in- 

 creased number of Polian vesicles and the location of the 

 longitudinal muscles that lie along and just internal to 

 the radial canals. About all one can say is that corre- 

 lated with a more complete radial symmetry with respect 

 to the Polian vesicles, there is a greater plasticity in the 

 retractor muscles, causing them to divide longitudinally 

 into separate muscle bands. 



To what extent, or how completely, may these indi- 

 vidual variations be reproduced in the process of regenera- 

 tion? An answer was obtained in the following way. 

 First a close examination was made of all parts eviscer- 

 ated and a record was kept of all organs showing variable 

 structures. Special attention was given to Polian vesicles 

 and to retracter muscles. The mutilated specimens were 

 then placed in separate aquaria in which the water was 

 changed frequently to prevent it from becoming stale. 

 After a considerable interval these animals were killed 

 and the regenerated organs were compared with the lost 

 parts. Table IV shows several individuals compared in 

 this way. The number of retractor muscles found in each 

 radius is given in the order of the radii taken in a clock- 

 wise direction. A study of the table indicates that there 

 is a strong tetndeney to reproduce individual peculiarities, 

 as shown by individuals B, E, G, H, M and 0. This does 



