404 
r. M. BALFOUR. 
prseoral ciliated ring of the Trochosphere may very probably 
be derived directly from the circumoral ciliated ring of the 
Pilidium ; the other ciliated rings or patches of the Trocho- 
sphere having a secondary origin. 
Fig. 21 . — Cyphonautes {Larva of Memhraiii'pora). (After Hatschek). m. 
mouth ; a, anus ; f.g. foot gland ; x. problematical body (probably a 
bud). 
The larva of the Brachiopoda x(fig. 6), in spite of its 
peculiar character, is, in all probability, more closely related 
to the Chsetopod Trochosphere than to any other larval type. 
The most conspicuous point of agreement between them is, 
however, the possession in common of provisional setae. 
Echinoderm larvae differ from the Trochosphere, not only 
in the points already alluded to, but in the character of the 
ciliated band. The Echinoderm band is longitudinal and 
postoral. As just stated, there is reason to think that the 
praeoral band of the Trochosphere and the postoral band of 
the Echinoderm larva are both derived from a ciliated ring 
surrounding the oral disc of the prototype of these larvae 
{vide fig. 8). In the case of the Echinodermata the anus 
must have been formed on the dorsal side of this ring, and 
in the case of the Trochosphere on the ventral side ; and so 
the difference in position between the two rings was brought 
about. Another view with reference to these rings has been 
put forward by Gegenbaur and Lankester, to the effect that 
the praeoral ring is derived from the breaking up of the single 
band of most Echinoderm larvae into the two bands found 
in Bipinnaria {vide fig. 10). There is no doubt a good deal 
is to be said for this origin of the praeoral ring, and it is 
strengthened by the case of Tornaria; but the view adopted 
above appears to me more probable. 
