356 C. M. Child 
the line of union being at a. Both the oral and aboral parts of 
the body-wall near the cut undergo more or less differentiation 
and growth and become much thinner as in other cases of wound 
closure, so that the region of union is marked by a band of trans- 
parent new tissue, which in section is much thinner than other 
parts of the body-wall (Fig. 4). 
ei 
Fics. 2-5 
By this method of closure the enteric cavity loses all connection 
with the exterior except such as may occur through the cinclides. 
Since the original oral end unites with the aboral endin suchcases 
the terms oral and aboral lose their significance so far as the piece 
as a whole is concerned, but it will be convenient in the following 
description to designate as oral and aboral respectively, those 
parts of the body-wall which originally formed the oral and aboral 
ends of the piece. ‘Thus in Fig. 4 the oral end lies just above 
the line of union at a and the aboral end just below. 
After closure the rings become more or less distended by the 
entrance of water through the body-wall, though in all cases the 
distension is, as in cesophagéal pieces, far below that of the normal 
animal. ‘They also perform certain rather remarkable movements 
which result in most cases in changing the position of the region of 
union so that it comes to lie somewhere near the equator on the 
outside of the ring (Fig. 5, a, a) instead of at the equator facing the 
central opening (Fig. 4, a) as originally. This movement may be 
described as a revolution of the tissues about a circular axis situ- 
