564 T. A. STEPHENSON 
sphincter, though not a very strong one; some developed 
suckers on the body-wall, and one curious animal formed 
special sphincters whereby it could cut off its tentacles at will— 
it also retained some primitive features (Boloceroides). 
The present-day forms which have gone no further than this 
are the Myonanthidae. 
A large number of forms, however, did go further, and attained 
a larger number of perfect mesenteries (cf. Text-figs. 16, H, 
and 10). Often the endodermal sphincter developed and 
sometimes became very strong, though some forms still remained 
sphincterless, or with very little or a moderate sphincter. 
Some of the advanced ones with strong sphincters have the 
tentacular and discal musculature embedded in the mesogloea. 
Among these forms the body either remained smooth, or 
developed verrucae or acrorhagi or both, but never vesicles. 
These are the Actiniidae s.s. in the sense taken on p. 546. 
To go back a little, from somewhere near the Myonanthidae 
arose a group of delicate forms which retained the six pairs 
of perfect mesenteries, but the body became divided into 
a scapus and eapitulum, and either from the scapus or from 
the region where scapus and capitulum jom (and sometimes 
above that region as well) there grew out hollow sac-like 
diverticula, often compound—the vesicles. Little or no 
sphincter was attained. These forms are the Alicidae. 
There is another set of forms with these vesicles, but with 
usually more numerous perfect mesenteries. They sometimes 
have a less delicate body, and occasionally mesogloeal tentacle- 
muscle. There is often a well-developed endodermal sphincter, 
but it may be weak or absent. Perhaps these, or some of them, 
arose, independently of the Aliciidae, from among the Actinudae, 
or perhaps they arose from near the Aliciidae by a mesenterial 
change. Whichever way it was, they represent onward steps. 
They are the Phyllactidae—a somewhat heterogeneous group 
to be further discussed in Part III. <A section of one of them, 
with many perfect mesenteries, is shown in Text-fig. 10. 
A form, or perhaps several forms, which from our hitherto 
incomplete knowledge of them would seem to have arisen near 
