No. r,f,o] 



REGENERATION 



animal; therefore the need of oxygen would be propor- 

 tionately greater than the supply, and the Thyone ren- 

 dered more susceptible to evisceration. Now while autot- 

 omy undoubtedly enables the animal to maintain its exist 

 ence for a considerable period on a smaller supply of 

 oxygen, the times when this would become necessary in 

 nature are probably rare, and it would be futile to specu- 

 late upon what evolution yet has in store for the process. 



According to Lang, the retractor muscles of the oral 

 region have been derived by the splitting up of the ori- 

 ginally simple longitudinal muscles, and this specializa- 

 tion became more marked as the oral tentacles became 

 more highly developed and required increasing protec- 

 tion. Species are to be found in the Dendrochirotae in 

 which the separation and branching off of retractors from 

 the longitudinal muscles has not yet been perfected. In 

 regeneration the retractor muscles of Thyone are derived 

 in the same way, i. e., by splitting off from the longitu- 

 dinal muscles, and such progress is made that they arc 

 fairly well developed by the time the tentacles take up the 

 function of feeding. The increasing sensitiveness and the 

 later activity of the regenerating animal are presumably 

 associated with the development of a new nervous system. 



If we may regard the bilateral echinoderm larva as 

 representing an early phylogenetic stage rather than a 

 larval adaptation to a free-swimming existence, we will 

 now discuss the symmetry of Thyone. As stated above, it 

 is generally agreed that the radial arrangement of parts 

 of the echinoderm body is due to a fixed stage in its 

 ancestral history. Some holotlmrians and spantangoids, 

 show in their ontogeny first a free stage, second a radial 

 stage, and finally a bilateral adult. During the develop- 

 ment of asteroids that have a fixed embryonic stage, the 

 early bilateral symmetry is soon disarranged by the 

 development of organs on the left side of the animal. 

 For example, the left hydrocele takes the form of an un- 

 closed water-vascular rosette which grows around the 

 esophagus to form the ring canal and its appendages, and 

 its connection with the dorsal pore gives rise to the stone 



