62 C. M. CHILD. 



wall. The longitudinal component must cause inrolling at the cut 

 end until either the local tension due to the formation of radial 

 folds in consequence of the in rolling or the mutual contact of 

 appressed portions of the inrolled wall opposes a resistance equal 

 to the elastic tension. It is probable that in the oral half of the 

 body, the mass of mesenteries and mesenterial filaments also 

 oppose more or less resistance to the inrolling margins. 



The flattening of the piece which often occurs (Fig. 3) is simply 

 the result of gravity. If the collapsed piece lies on one side 

 during several hours the weight of the body-wall is sufficient to 

 bring about the flattening to a greater or less extent. In con- 

 sequence of the flattening the openings at the ends are frequently 

 elongated in the plane of flattening and slit-like, the inrolling 

 occurring chiefly on the two margins of the slit. 



In pieces from the extreme aboral end the inrolling at the cut 

 margin may proceed much further (Figs. 4-6). Here the body- 

 wall and especially the muscular layer is much thinner and must 

 offer much less resistance to the elastic tension. Moreover, the 

 enteron is practically empty in this part of the body. In con- 

 sequence of these conditions the inrolling may proceed so far 

 that portions of the margin are directed orally (Fig. 6). 



The closure of a lateral cut by bending of the whole piece 

 (Figs. 79) especially resembles a definite adaptive reaction, but 

 can be explained as the result of elastic contraction. A cylindri- 

 cal piece such as Fig. I does not become bent or curved so long 

 as elastic tension on opposing sides of the body is equal. If in 

 any way the tension on one side be reduced in effectiveness the 

 body must bend toward that side. A transverse cut through the 

 body-wall on one side, or the removal of a piece as in Fig. 7 

 interrupts the continuity of the body-wall. The longitudinal 

 component of the elastic tension acts on the parts above and 

 below the cut and causes contraction and inrolling of their edges. 

 But by removal of a piece of the body-wall an open space is left 

 and the longitudinal component of tension on the opposite side 

 of the body causes bending of the piece so that the concave sur- 

 face is on the side of the cut. The larger the piece cut out from 

 the one side the greater will be the bending since it will continue 

 until contact between the cut margins affords a resistance equal 

 to the opposing tension. 



