44 SCIENCE PROGRESS. 



as those of any Gastropod — in Nucula to the extent even 

 of having separate cerebro-pedal and pleuro-pedal connec- 

 tives (18, 19) — it seems profitless to pursue these ill- 

 balanced speculations any further. 



The utmost ingenuity cannot overcome the fact that 

 there is a fundamental disparity between the Turbellarian 

 and Molluscan body. This disparity is revealed by em- 

 bryology ; but to embryology Thiele pays scant attention. 

 Thiele's argument is practically this (24, p. 504), — that 

 the only route from Ccelenterates to Bilateralia is via the 

 Ctenophores to Polyclads, and that Annelids and Molluscs 

 are consequently to be derived from Polyclad ancestors. 

 Embryology seems to me, however, to point to two lines of 

 descent at least, from the Ccelenterates to the Bilateralia. 

 In each case the oral surface of the Ccelenterate ancestor 

 became the ventral surface of the Bilateral descendant ; but 

 along one line of descent the primitive mouth or blastopore 

 retained its ancestral form as a simple circular orifice in the 

 middle of the ventral surface, and opened into a gastral 

 cavity devoid of an anal orifice (Polyclads) ; while along the 

 line of descent which led to the Annelida and Mollusca the 

 blastopore elongated along the ventral surface, as Sedg- 

 wick has so ably contended, its lips coalesced except at the 

 two extremities, and these open ends constituted the mouth 

 and anus of the Ccelomate descendants. Thiele has 

 altogether overlooked the significant behaviour of the blas- 

 topore in Annelidan and Molluscan embryos ; and since 

 no similar modification of the blastopore is known in the 

 case of Turbellarians and Trematodes, in which groups the 

 absence of an anus is so marked a characteristic, we are 

 amply warranted, I think, in drawing the conclusions which 

 I have emphasised above. 



The admission of this distinction is however fatal to 

 any theory of the Polyclad ancestry of the Mollusca. The 

 foot of the Mollusca is a development of the fused lips of 

 the elongated blastopore, and can in no case be homo- 

 logised with the ventral sucker of Turbellarians which lies 

 entirely behind the blastopore. The same remark applies 

 to Lang's comparison of the Molluscan foot with the ventral 



