6 SCIENCE PROGRESS. 
tiation of the nerve-centres which supplied the primary 
pallium, vzz., from the lateral or epipodial centres. So far 
from this being the case, however, Thiele himself (xxv., pp. 
587-9) adopts the view that the pallial nerves as well as the 
pleural ganglia of Gastropoda are secondary derivatives of 
the ventral or pedal cords. 
The recklessness of Thiele’s comparisons reaches its 
high-water mark, perhaps, in his remarks on the nervous 
system of Pelecypoda. Correlated with the existence of 
numerous sense-organs (eyes, tentacles, etc.) along the 
mantle edge, there exists in many forms (Avca, Pecten, 
Pinna, etc.) a nervous ring around the mantle which may 
take the form either of a complete ring of peripheral ganglia 
united by a plexus, or of a circumpallial ganglionated nerve, 
as was recognised by Duvernoy (5) more than thirty years 
ago. Since the mantle-lappets of the two sides of the body 
unite posteriorly above the anus, this pallial nerve-ring lies 
above the gut. The ring is connected with the cerebro- 
pleural ganglia by means of the anterior pallial nerves, and 
with the visceral (parieto-splanchnic) by means of branches 
from the great posterior pallial nerves. Accordingly Thiele 
homologises the circumpallial nerve-ring with the lateral 
cords of C/zton and with the epipodial plexus of the Rhi- 
pidoglossa. 
The first of these homologies seems not unreasonable, for 
no one disputes the homology between the mantle of Chztox 
and that of Pelecypoda. Moreover Kowalevsky’s discovery 
that Cz¢on in its later embryonic phases is provided with 
a pair of transitory eyes which lie outside the velar area 
and have some close connection with the lateral nerve- 
cords, renders this comparison particularly worthy of 
attention. But how the circumpallial nerve of Pelecypoda 
can be in any sense homologous with the epipodial plexus 
of Gastropoda, when the latter structure lies beneath the 
gut and has no connection with the cerebral ganglia, either 
directly or by the intermediation of the pleural ganglia, it 
