THe MORPHOLOGY OF THE MOLLUSCA. 21 
be as follows. In the evolution of the Annelida the proto- 
troch and nerve-ring remained for a long time unmodified, 
and did not share in the elongation of the postero-ventral 
region of the body which gave rise to the trunk of the 
Annelid. This would explain the absence of the dorsal 
nerve-ring in the adult Annelid, provided that the nerve- 
ring, together with the prototroch, came to have merely a 
larval significance,—as actually happens in the ontogeny of 
Annelids to-day. On the other hand, in the evolution of 
the Mollusca from the same simple type of ancestor, the 
whole body must have shared in the elongation—the proto- 
troch and nerve-ring as well as the more ventrally placed 
parts of the body. This elongated nerve-ring I identify 
with the pleuro-visceral ring of Amphineura, although the 
phyletic connection between the nerve-ring and the ciliated 
band is inferred from the development of certain Gastro- 
pods rather than from the Amphineura themselves. Asa 
larval adaptation for conveniences of natation I imagine 
that a separation became gradually effected in embryonic 
life between the ciliated ring and the nerve-ring, the former 
becoming restricted to the anterior end of the larval body, 
while the latter became more and more extended far? passu 
with the elongation of the trunk. Such a separation is to 
some extent paralleled in the development of Holothurians 
from the Auricularia larva, as described by Semon. On 
this theory alone can I explain to myself the absence of the 
ancestral nerve-ring in the trochospheres of Mollusca, and 
I find some support for this view in the ontogeny of Nemer- 
tines. The lateral nerve-cords in this group have the same 
relation to the gut and brain as have the pleuro-visceral 
cords of Chz¢on, since they form a dorso-lateral ring, the 
posterior commissural portion passing above the rectum. 
In Nemertines there can be very little doubt that this 
nerve-ring has been derived phyletically by the elonga- 
tion of a nerve-ring which underlay the ciliated band of a 
more or less P2lzdium-like ancestor, as it underlies the 
