FORM-REGULATIONS IN CEK 1 AN TI I L'S. 285 



antlius. If we consider animals moving about, oral end in ad- 

 vance as they very commonly do, a cylindrical portion of the body 

 situated at any region except the extreme end, is, let us assume, 

 subjected to a certain degree of tension in consequence of the 

 movement. Now since this piece is attached to other parts both 

 at its oral and aboral ends about its whole circumference the 

 strains to which its tissues are subjected are almost wholly par- 

 allel to the longitudinal axis. If now the piece be removed from 

 the whole and allowed to regenerate in the typical manner closure 

 of the ends and outgrowth of new tissue at the aboral end occur 

 before the piece begins to move about. If now the piece begins 

 to creep and is in consequence subjected to tension the elements 

 of this tension affecting various parts will differ not only in 

 amount but also in direction because the piece is no longer at- 

 tached to other parts at both ends by its whole circumference. 

 Comparing a long and a short piece it is clear that the change in 

 direction of the strains in the long piece considered as a whole 

 is very much less than in the short piece. We may expect, 

 therefore a greater change of form in the short piece than in the 

 long piece, after equilibrium is attained. The greater change in 

 the piece C as compared with A, B, and D, which I am inclined 

 from comparison with other pieces to regard as real, may then 

 be explained on the same basis as similar changes in other creep- 

 ing forms. The fact, shown by the figures and table, viz., that 

 the change of form became much more rapid toward the end of 

 the experiment, i. e., in the fifth and sixth weeks, agrees well with 

 the fact that the piece did not begin to move about freely during 

 the first month after section. During this time it never climbed 

 out of the jar, but during the next two weeks it climbed out of a 

 deep jar into the large tank almost daily and wandered about the 

 tank. As often as found outside it was returned to the jar. It 

 was during this time that the chief change of form occurred (com- 

 pare Figs. 17 and 18). These facts may perhaps indicate that 

 the movements are more important than the internal pressure in 

 bringing about the change of form. 



In Cerianthus then we find another case in which the assump- 

 tion of a mysterious formative principle is in no way necessary. 

 Form in the lower animals is probably to be considered as rela- 



