LARVAL FORMS. 381 



geous to a free-swimming form, is sufficient to shew that this 

 supposition is by no means extravagant ; while the presence of 

 only two alimentary diverticula in Echinoderm larvae 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 

 diverticulum 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 Echino- 

 derms 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 ali- 

 mentary 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 larvae had 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 archetype was probably 

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

 Pilidium type and of the Echinodermata was most likely derived. 

 The ciliated ring of the Pilidium varies greatly in its characters, 



