ihe MORPHOLOGY OF Tit MOLEUSCA. 7 
is altogether impossible to conceive. And this is not all. 
The posterior connection between the circumpallial nerve 
of Pelecypoda and the visceral ganglia is compared by 
Thiele with the posterior connectives between the lateral 
and ventral cords of Amphineura; and the time-honoured 
visceral nerve-cords of Pelecypoda, with the visceral (parieto- 
splanchnic) ganglia upon them, are homologised with the 
ventral cords of the Amphineura. To reveal the absurdity 
of these comparisons it is sufficient, | think, to remind my 
readers that the ventral cords of Czéox are concerned ex- 
clusively with the innervation of the musculature of the 
foot; while the visceral cords of Pelecypoda innervate 
the body-wall, ctenidia and viscera, in addition to the 
posterior adductor muscle. How these supposed homo- 
logues of the ventral cords of Chzton have come to assume 
so many of the functions of the lateral or pallio-visceral 
cords, is not explained; and since Pelecypoda possess a 
pair of pedal ganglia in the foot, as typical in their relations 
as those of any Gastropod—in Mucula 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 v2@ 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 
