374 ORIGIN OF PILIDIUM LARVA. 



larva becoming more elongated, and by the region between the 

 mouth and one end of the body becoming the prreoral 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 resem- 

 blance to a simplified free-swimming 

 Ccelenterate (Medusa), and that the 

 conversion of such a radiate form into 



4-U U'l - 1*1 1 t U i1_ FlG. 226. POLYGORDIUS 



the bilateral took place, not by the LARVA- ( After Hatschek.) 

 elongation of the aboral surface, and ^ mouth; ^ ^.^ 



the formation of an anus there, but by phageal ganglion ; npk. nephri- 

 ,1 11 , f , i i r dion : me.p. mesoblastic band ; 



the unequal elongation of the oral face, an a ' nus ( oL stomach . 



an anterior part, together with the dome 



above it, forming a praeoral 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 Ccelenterata and the Turbellaria 1 , and shews, if true, 

 that the ventral and median position of the mouth in many 

 Turbellaria is the primitive one. 



The above suggestion as to the mode of passage from the radial into the 

 bilateral form differs largely from that usually held. Lankester 2 , for 

 instance, gives the following account of this passage : 



" It has been recognised by various writers, but notably by Gegenbaur 

 and Haeckel, that a condition of radiate symmetry must have preceded the 

 condition of bilateral symmetry in animal evolution. The Diblastula may 

 be conceived to have been at first absolutely spherical with spherical 

 symmetry. The establishment of a mouth led necessarily to the establish- 

 ment of a structural axis passing through the mouth, around which axis the 

 body was arranged with radial symmetry. This condition is more or less 

 perfectly maintained by many Ccelenterates, and is reassumed by degrada- 



1 Vide Vol. II. pp. 179 and 191. In this connection attention may be called 

 to Cceloplana Melschnikowii, a form described by Kowalevsky, Zoologischer Anzeiger, 

 No. 52, p. 140, as being intermediate between the Ctenophora and the Turbellaria. 

 As already mentioned, there does not appear to me to be sufficient evidence to prove 

 that this form is not merely a creeping Ctenophor. 



" Quart. Journ. of Micr. Science, Vol. XVII. pp. 422-3. 



