LARVAL FORMS. 383 



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 that of the affinities of the 

 other larval types to these two types. 



The most important of all the larval types is the Trochosphere, 

 and this type is undoubtedly more closely related to the Pilidium 

 than to the Echinoderm larva. Mitraria amongst the Chaetopods 

 (fig. 233) has, indeed, nearly the form of a Pilidium, and mainly 

 differs from a Pilidium in the possession of an anus and of 

 provisional bristles ; the same may be said of Cyphonautes (fig. 

 234) amongst the Polyzoa. 



The existence of these two forms appears to shew that the 

 praeoral 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 Trochosphere having a 

 secondary origin. 



The larva of the Brachiopoda (fig. 220), in spite of its peculiar 

 characters, is, in all probability, more closely related to the 

 Chaetopod 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. 231). 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 

 \\iQventralside; 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 of the Trochosphere is derived 

 from the breaking up of the single band of most Echinoderm 

 larvae into the two bands found in Bipinnaria (vide fig. 223) and 

 the atrophy of the posterior band. There is no doubt a good 

 deal to be said for this origin of the proeoral ring, and it is 



