Form Regulation in Harenactis attenuata 357 
ated in the enteric cavity of the ring. It undoubtedly results from 
differences in the degree of contraction of different parts of the 
body-wall, and since in most cases it accomplishes a more or less 
definite and characteristic result, I am inclined to believe that it 
is really an attempt at orientation, though of course anything like 
normal orientation is impossibie. At any rate the fact remains 
that most of the pieces assume the position indicated in Fig. 5 and 
this position is usually retained, although stimulation may bring 
about a temporary return to the original position or may change 
the position of the region of union to the upper or lower surface. 
Such changes are usually followed by a return to the position of 
Hig. 5. 
2 The Course of Restitution in Rings 
Restitution in these rings consists, briefly stated, in the forma- 
tion of single tentacles, of irregular groups of tentacles, or of small 
discs with complete circles of tentacles varying in number. The 
tentacles may form on both sides of the line of union, 1. e., they 
may be both oral and aboral or they may appear wholly on the 
oral side of the line of union. In the latter case discs with com- 
plete circles of tentacles are usually formed, though these some- 
times appear when the tentacles are of both oral and aboral origin. 
The number of tentacles or groups formed varies greatly in differ- 
ent cases and is apparently indefinite. 
It will be convenient to consider the data under two heads, viz: 
those cases in which tentacles are formed on both sides of the 
line of union and those in which they appear only on one side. 
The following figures of the rings are on a larger scale than pre- 
ceding figures. In all cases where the line of union is visible in 
the aspect shown in the figure its position is approximately indi- 
cated bya dotted line. In the specimens the region of union is not 
sharply marked off from other regions but is distinguishable as a 
band of transparent new tissue of varying width, in the formation 
of which both the oral and aboral end of the body-wall have taken 
part. The line of union indicated by the dotted line inthe figures 
represents as nearly as possible the middle of this band. In many 
of the figures the tentacle-groups are represented with the discs 
