child] form-regulation in ccelentera and turbellaria 139 



diffusion has been established : there remain only waste products, 

 debris, etc. Now, as the piece decreases in size a point is reached 

 where the intestinal space is reduced to such an extent that the 

 residual fluid fills it completely and begins to exert a pressure upon 

 its walls. The changes following this period are most remarkable: 

 from the sides of the axial intestine a new set of lateral branches 

 begins to grow. These are typical intestinal branches, not ruptures 

 or masses of disintegrating cells. The entrance into and exit from 

 these branches of the residual fluid can be clearly observed. More- 

 over, they are not the old branches : it might be believed that the 

 old branches had not actually disintegrated, but had merely become 

 invisible as the result of collapse and were now become visible 

 again because distended. This, however, is not the case ; these new 

 branches are less numerous and farther apart and much more deli- 

 cate than the old branches, but they are unmistakably intestinal 

 branches. This formation of a new set of intestinal branches was 

 observed repeatedly in pieces of the kind described, which were kept 

 for several months. It is of considerable importance as indicating 

 how closely typical structure of this organ is dependent upon its 

 typical function. So close is this dependence that typical structure 

 cannot continue to exist when the typical function is abolished. One 

 further point may be merely suggested here ; viz., the possibility that 

 the functional stimulus may be a mechanical tension exerted on the 

 intestinal walls by the fluid contents. 



III. Cerianthus 



The body of Cerianthus is elongated and almost cylindrical in 

 form, except aborally where it tapers to a blunt point. The course 

 of regeneration, and indeed the possibility of regeneration, is de- 

 termined to a large extent by the shape of the piece. In pieces cut 

 from the body the body-wall soon begins to roll inward at the cut 

 surface, this change being due not to muscular contraction, but to the 

 elasticity of certain layers of the body-wall, apparently the mesogloea, 

 at least in large part. In cylindrical pieces inrolling of the ends 

 only occurs and typical regeneration is possible. In pieces slit 

 longitudinally or in strips cut from the body the inrolling may often 

 be spiral or transverse and typical regeneration impossible. 



In cylindrical pieces regeneration is typical and complete, resulting 

 in a new individual of smaller size than the original and differing 

 from it more or less widely in form. Complete regeneration is 

 possible in pieces which comprise only a small fraction of the whole, 

 provided they are in the form of rings of tissue. Such pieces, about 



