308 



ORIGIN OF PILIDIUM LARVA. 



lavva is in some important respects less highly differentiated than the 

 larviB of the five other groups. It is, in the first place, without an 

 anus ; and there are no grounds for supposing that the anus has 

 become lost by retrogressive changes. If for the moment it is granted 

 that the Pilidium larva represents more nearly than the larva? of the 

 other groups the ancestral type of larva, what characters are we led 

 to assign to the ancestral form which this larva repeats ? 



In the first place, this ancestral 

 form, of which fig. 231 A is an 

 ideal representation, would appear 

 to have had a dome-shaped body, 

 with a flattened oral surface and 

 a rounded aboral surface. Its 

 symmetry was radial, and in the 

 centre of the flattened oral surface 

 was placed the mouth, and round 

 its edge was a ring of cilia. The 

 passage of a Pilidium-like larva 

 into the vermiform bilateral Platy- 

 elminth form, and therefore it 

 may be presumed of the ancestral 

 form which this larva repeats, is 

 effected by the larva becoming 

 more elongated, and by the region 



between the mouth and one end of the body becoming the praeoral 

 region, and by an outgrowth between the mouth and the opposite 

 end developing into the trunk, an anus 

 becoming placed at its extremity in the 

 higher forms. 



If what has been so far postulated is 

 correct, it is clear that this primitive larval 

 form bears a very close resemblance to a 

 simplified free-swimming Coelenterate (Me- 

 dusa), and that the conversion of such a 

 radiate form into the bilateral took 

 place, not by the elongation of the aboral 

 surface, and the formation of an anus there, 

 but by the unequal elongation of the oral 

 face, an anterior part, together with the 

 dome above it, forming a prseoral lobe, and 

 a posterior outgrowth the trunk (figs. 226 

 and 233); while the aboral surface became 

 the dorsal surface. 



This view fits in very well with the anatomical " resemblances 

 between the Coelenterata and the Turbellaria^ and shews, if true, 



Fig. 225. Two Ch^topod larv^. 

 (From Gegenbaur.) 



o. mouth; i. intestine; a. anus; v. 

 prseoral ciliated band ; w. perianal cilia- 

 ted band. 



Fig. 226. Polygokdius 

 LAKVA. (After Hatschek.) 



m. mouth ; sg. supra-oeso- 

 phageal ganglion ; nph. ne- 

 phridion ; vie.p. mesoblastic 

 band; an. anus; ol. stomach. 



^ Vide Vol. I. pp. 148 and 158. In this connection attention may be called to 

 Cceloplana Metschnikowei, a form described by Kowalevsky, Zoologischer Anzeiger, No. 



