508 T. A. STEPHENSON 
placmg Bolocera with Boloceroides, but is additional 
evidence for thinking of the former as an Actiniid, taking the 
view that will be developed below, that the Actiniidae are 
one of the next steps on from the Myonanthidae. 
IT am conscious that the arguments used in this section are 
rather dangerous, and that along some such line an attack 
might be developed upon the whole system of classification 
by summation of characters. But I feel that it is a special 
case, like one or two others, and that, as suggested in Part I 
(p. 470), the summation principle must not be used blindly 
like an arithmetical measure; looking upon it as useful 
typically, but needing modification here and there. 
§C. The Ptychodactidae. 
Carlgren (1911) has shown clearly that two curious genera, 
very different in detail but similar in fundamentals (Pt ycho- 
dactis and Dactylanthus), should be thought of together 
as forming one family. The debatable ground here is as to 
where the family fits into the general scheme. Carlgren includes 
it in his Protantheae with the Gonactiniidae. That the 
Ptychodactidae must be kept apart from the ordinary Actinians 
is pretty clear; also that they must come next to the Gonac- 
tinids in a list. But apart from this general location, they 
seem to have very little to do with the Gonactinids, and 
should be marked off from these by being placed in a group 
of their own and of higher rank than a family. 
Of primitive characters they share with Gonactinuds the 
following : absence of basilar muscles although there is a base ; 
similarity of structure between tentacles and body-wall— 
spirocysts and ectodermal muscle in both ; sphincter little or 
none; mesenterial musculature weak, hardly forming retractors. 
They have no ciliated tracts on the filaments. On the other 
hand they have diverged from the Gonactiniuds as regards 
size—they can get quite large—and have attained not only 
pairs of perfect mesenteries but often a good many of them. 
Ptychodactis has become very broad and has almost lost 
its actinopharynx (a unique case), and has numerous tentacles 
