Form Regulation in Harenactis attenuata 363 
c, and d the middle tentacles arise on one side of the line of union, 
probably the oral side, if this case is similar to others, though there 
is no visible distinction between oral and aboral regions in this 
piece except these tentacles. In group a one tentacle arises 
on one side of the line of union, one on the other and two from 
the line of union. Fig. 16 shows this group of tentacles in surface 
view, the line of union being approximately indicated by the 
broken line. 
No more tentacles were produced and ten days later the piece 
was almost wholly collapsed and the tentacles were reduced to 
slight elevations: death occurred a few days later. 
This case is not very different from No. III. The only differ- 
ence of importance is that in the one group where the tentacles 
form a complete circle they all arose at the same time and are of 
equal length, although they are partly from the original oral end 
and partly from the aboral, 1. e., in this case the difference in 
time of appearance of tentacles on the two sides of the line of union, 
which was apparent in the preceding cases, is not present. 
V_ Figs. 17 to 20. ‘The piece was taken from aboutthe region 
c, d, Fig. 1: Parts of the mesenterial organs were extruded and 
cut off, but considerable portions remain. Fig. 17 shows a stage 
twelve days after section: the line of union is at the equator. 
Three groups of tentacles have appeared, each forming a com- 
plete circle: group a has six tentacles, group ) four and group c 
three. Figs. 18 to 20 are surface views of the three groups and 
also show the relations between the mesenteries and the tentacles: 
the line of union is indicated by a dotted line. Discussion of the 
mesenterial relations is postponed to a later section, but attention 
may be called here to the radial arrangement of the mesenteries 
in the regions of tentacle groups and to the fact that tentacles 
apparently form as readily between the oral end of one mesentery 
and the aboral end of another as between two oral ends. The 
two lateral tentacles in Fig. 18, the two long tentacles in Fig. 19 
and the right lower and left upper tentacle in Fig. 20 arise between 
oral and aboral ends of mesenteries. These figures are given 
merely as illustrations of the mesenterial relations, not because 
these groups differ in any way from those already described. In 
