Form Regulation in Harenactis 71 
In the region distal to the incision, however, the new hetero- 
morphic tentacles arise in the lumen of the cesophagus (Fig. 8). 
It is evident from the figure that their origin at this point does not 
constitute a change from the usual relations of tentacles to the 
line of union. ‘They arise at or rather very near the aboral end of 
this partially isolated region of the body-wall exactly as in other 
cases described (Figs. 2 and 5). In cases of this kind the tips 
of these aboral tentacles can often be seen protruding from the 
lateral mouth when the animals are fully extended and distended, 
though they sometimes extend down the cesophagus and are not 
visible at all from the exterior. Under these conditions the hetero- 
morphic tentacles do not usually attain any great length. ‘This 
failure to develop is undoubtedly due to the fact that the pressure 
of other parts upon them prevents their distension and conse- 
quent growth. When the animal is distended the region proxi- 
mal to the incision presses upon the aboral end of the distal region 
and the walls of the cesophagus are usually more or less closely 
applied to each other, so that conditions for the growth of tentacles 
are very unfavorable. In a number of cases I have seen these 
aboral tentacles gradually crushed out of existence. 
The second case of this kind to be described is shown in Figs. 
gto 11. Here invagination has proceeded so far before the inci- 
sion was completed that the disc instead of the cesophagus was 
involved (Fig. 9). Fig. 10 indicates the relations of parts after 
union had occurred and the animal had become distended again. 
Distal to the incision the cut aboral surface of the body-wall had 
united with the cut surface of the disc so that the line of union 
was at a (Fig. 10). Proximal to the incision union between 
body-wall and cesophagus occurred at b. 
As in the preceding case the tentacles appear in relation to the 
cut surfaces of the body-wall, but in the case of the oral partial 
disc proximal to the incision regulation gradually brings the parts 
into the usual positions (Fig. 11). In the region distal to the in- 
cision, however, the heteromorphic tentacles arise between the 
old tentacles and the mouth (Fig. 11) on what is apparently the 
disc, so far as its position is concerned, though it is actually the 
aboral end of the partially isolated region of the body-wall, as is 
