LARVAL FORMS ; THEIR NATURE^ ORIGIN AND AFFINITIES. 403 
pore ceases to be a difficulty. As may be seen, by referring 
to fig. 18 c, the anus is placed on the dorsal side of the 
ciliated band. This position for the anus adapts itself to the 
view that the Echinoderm larva had originally a radial sym- 
metry, with the anus placed at the aboral apex, and that the 
present terminal position of the anus arose with the elonga- 
tion of the larva on the attainment of a bilateral symmetry. 
It may be noticed that the obscure points connected with 
the absence of a body-cavity in most adult Platyelminthes, 
which have already been dealt with in my essay on the 
germinal layers, crops up again here ; and that it is necessary 
to assume either that alimentary diverticula, like those in 
the Eohinodermata, were primitively present in the Platy- 
elminthes, but have now disappeared from the ontogeny of 
this group, or that the alimentary diverticula have not 
become separated from the alimentary tract. 
So far the conclusion has been reached that the archi- 
type of the six types of larvae was a radiate form, and that 
amongst existing larvae it is most nearly approached in 
general shape and in the form of the alimentary canal by 
the Pilidium group, and in certain other particulars by the 
Echinoderm larvae. 
The edge of the oral disc of the larval architype was pro- 
bably armed with a ciliated ring, from which the ciliated 
ring of the Pilidium type and of the Echinodermata were most 
likely derived. The ciliated ring of the Pilidium varies 
greatly in its characters, and has not always the form of a 
complete ring. In Pilidium proper (fig. 19 a) it is a simple 
ring surrounding the edge of the oral disc. In Miiller’s 
larva of Thysanozoon (fig. 9 b) it is inclined at an axis to 
the oral disc, and might be called praeoral, but such a term 
cannot be properly used in the absence of an anus. 
The Echinoderm ring is oblique to the axis of the body, 
and, owing to the fact of its passing ventrally in front of the 
anus, must be called postoral. 
The next point to be considered is the affinities of the 
other larval types to these two types. 
The most important of all the larval types is the Tro- 
chosphere, and this type is undoubtedly more closely related 
to the Pilidium than to the Echinoderm larva. Mitraria 
amongst the Chsctopods (fig. 20) retains, indeed, the form 
of Pilidium very closely, and mainly differs from a Pilidium 
in the possession of an anus and of provisional bristles ; 
the same may be said of Cyphonautes (fig. 21) amongst the 
Polyzoa. 
The existence of these two forms appears to show tliat the 
