li:^!'^] Esscnhcrg: Distribution of the Polynoidae 217 



uew environment. A muscular adaptation occurs. The muscles, espe- 

 cially those in connection with locomotion supporting the parapodia, 

 are constantly used by the free-living worm and through this exercise 

 they are naturally well developed. In a commensal polynoid the 

 locomotor muscles are of no use and begin to degenerate. Secondly, 

 the dwelling place is too narrow and the walls of the tube press upon 

 the broad lateral surfaces. This constant pressure forces the muscles 

 to contract laterally and to expand ventrally, dorsally and longi- 

 tudinally where no pressure or resistance is met. If the commensal 

 pol.ynoid remains in this condition for some time, the shape of its 

 body will naturally be changed from a compressed flat one to a 

 rounded one. The elongation of the segments will increase the length 

 of the worm, the parapodia become shorter and broader and we have 

 a worm that has changed its shape so greatly that it could hardly be 

 recognized as being an individual of the species. 



That Polynoidae tend to retain the plasticity, which is common to 

 some higher types in the very early stage of development, is proved 

 by the great power of regeneration which they possess throughout 

 their lives. If a polynoid loses some of its segments, elytra or append- 

 ages, it regenerates them within a few weeks. This is an indication of 

 plasticity and adaptability to diverse conditions. 



The changes of color are probably produced by some enzymes pro- 

 duced by the messmate of the commensal polynoid. The small size of 

 the elytra is evidently due to the disuse and the degeneration of those 

 structures. The absence of the chitinous bulbs and protuberances on 

 the elytra may be accounted for in the same way. First, the free- 

 living polynoid is under the influence of some chemicals which act as 

 stimulants in the production of the bulbs on the elytra. Living as 

 a commensal its environment has been changed and the secretions or 

 enzymes of the messmate may impede the development of the rough 

 structures on the scales of the polynoid. That this is true is proved 

 by the fact that the first pair of elytra remain unchanged. The 

 reason for this condition is apparent. The polynoid is hidden in the 

 tube with its anterior end, its tentacles and head projecting. The 

 first pair of elytra remain in the same environment in the open water 

 as they were before the polynoid entered the tube hence they remain 

 unchanged. The external changes in this case may be traced back 

 to the changes of environment and to the quick response or the plas- 

 ticity of the polynoid undergoing these changes. 



