504 T. A. STEPHENSON 
contrast between the kind of filament with ciliated tracts 
and that without may be seen from Text-fig. 17, where three 
of the four sorts of filament illustrated have the tracts (though 
not all the same kind of tract, in detail), and the fourth has 
none (C). 
(u) Presence or absence of ectodermal muscle 
in body-wall.—In this case we are dealing with a universal 
ancestral character which has been allowed to die out in most 
forms. It persists in those retaining most primitiveness, and 
is present, at least partially or as a vestige, here and there 
among more advanced forms, physiological causes probably 
accounting for its retention. It can therefore only be used 
in a limited way in a classification—useful in defining primitive 
groups, but not a criterion of relationship when it becomes 
a question of forms some of which have retained it, in greater 
or less degree, and others have shed it. 
(iu) Presence or absence of spirocysts in ecto- 
derm of body-wall.—tThis is another character about 
which a similar view may be taken to that developed in con- 
nexion with the last one. 
(iv) Presence or absence of basilar muscles.— 
These muscles are natural developments correlated with the 
stabilizing of a well-marked basal disc. Their presence is 
certainly a good characteristic of the higher forms in general, 
but here again it may be misleading to think too much about 
them in connexion with transitional forms or forms of doubtful 
relationships. For purposes of family-definitions, it appears 
that the presence or absence of the base itself is the first 
consideration, basilar muscles or not. 
(v) Presence or absence of any perfect meta- 
cnemes.—One set of forms (Gonactinia, Protanthea, 
and Oractis) seem well distinguished from others by virtue 
of the fact that they alone among Actinians (excluding 
Edwardsiids and odd individuals among Halcampas, 
Aiptasias, &c.) have the four couples of protocnemes 
(the eight ‘ Edwardsia-mesenteries ’) perfect, none of the 
metacnemes being so, with the result that there are no perfect 
