308 



U RIG IX OF P1LID1UM LARVA. 



larva is in some important respects less highly differentiated than the 

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

 and there are no grounds for supposing that the anus has 



anus 



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

 that the Pilidium larva represents more nearly than the larvae 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 

 rounded aboral surface. Its 



FIG. 225. Two CHJETOPOD LARV^K. 

 (From Gegeiibaur.) 



o. month; /. intestine; a. anus ; v. 

 pra?oral ciliated band ; w. periaual cilia- 

 ted baud. 



a 



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 prseoral 



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 Ccelenterate (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. 220 

 and 233); while the aboral surface became 

 the dorsal surface. 



This view fits in very well with the anatomical' resemblances 

 between the Ccelenterata and the TurbeUaria 1 , and shews, if true, 



lipk 



7>ie.p 



an 



FIG. 226. POLYGORDIUS 

 LAKVA. (After Hatschek.) 



m. mouth ; sg. supra-osso- 

 phageal ganglion ; iiph. ne- 

 phridion ; me.p. mesoblastic 

 baud; . anus; ol. stomach. 



1 Vide Vol. i. pp. 148 and 158. In this connection attention may be called to 

 Cceloplana Mitxcl/nikowt'i, a form described by Kowalevsky, Zooloyischer Anzeiger, No. 



