314 PRIMITIVE RADIAL SYMMETRY OF EC HI NO DERM AT A. 



diverticula in Echinoderm larvce is quite in accord with the presence 

 of a single pair of perigastric chambers in the early larva of Actinia, 

 though it must be admitted that the derivation of the water-vascular 

 system from the left divertieulum is not easy to understand on this 

 view. 



A difficulty in the above speculation is presented by the fact of 

 the anus of the Echinodermata being the permanent blastopore, and 

 arising prior to the mouth. If this fact has any special significance, 

 it becomes difficult to regard the larva of Echinoderms and that of 

 the other types as in any way related ; but if the views already urged, 

 in a previous section on the germinal layers, as to the unimportance of 

 the blastopore, are admitted, the fact of the anus coinciding with the 

 blastopore ceases to be a difficulty. As may be seen, by referring to 

 fig. 231 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 Echino- 

 derm larva had originally a radial symmetry, with the anus placed at 

 the aboral apex, and that, with the elongation of the larva on the 

 attainment of a bilateral symmetry, the aboral apex became shifted 

 to the present position of the anus. 



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 the section of this chapter devoted to the germinal layers, present 

 themselves again here ; and that it is necessary to assume either that 

 alimentary diverticula, like those in the Echinodermata, were primitively 

 present in the Platyelminthes, 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 archetype of the 

 six types of larvse had a radiate form, and that amongst existing larva? 

 it is most nearly approached in general shape and in the form of the 

 alimentary canal by the Pilidium group, and in certain other parti- 

 culars by the Echinoderm larva 1 . 



The edge of the oral disc of the larval archetype was probably 

 armed with a ciliated ring, from which the ciliated ring of the Pili- 

 dium type and of the Echinodermata was 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. 232 A) 

 it is a simple ring surrounding the edge of the oral disc. In Miiller's 

 larva of Thysanozoon (fig. 222 B) it is inclined at an axis to the 

 oral disc, and might be called prseoral, 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 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 



