22 SCIENCE PROGRESS. 
ciliated band of the P2dedium-larva, although this phyletic 
origin is disguised by the profound metamorphosis which 
breaks the continuity of the ontogenetic record in Nemer- 
tines. On this theory of course the lateral cords of Nemertines 
do not correspond to the ventral cords of Annelids. The 
latter are represented by the general ventral plexus of 
Nemertines and by the pedal plexus or cords of Mollusca. 
These ventral nervous systems appear to bear relations to 
the dorso-lateral ring-nerve similar to those of the subum- 
brellar plexus of Medusa to the circumferential nerve- 
ring. 
It will be recognised from these remarks that the 
conclusions to which I have arrived present distinct points 
of agreement with those of Balfour (1, p. 378) and Sedg- 
wick (21) on the same subject, although attained throughout 
by an independent series of inductions. With both these 
writers I agree in tracing back the Molluscan nervous 
system to a primitively annular type, such as might be 
expected to exist in) a Coclenterate -ancestom | Balioun 
derives the whole Molluscan nervous system from a 
peripheral nerve-ring which followed the course of a hypo- 
thetical ciliated ring ; Sedgwick derives it from a broad 
plexus surrounding an elongated blastopore, such as occurs 
in existing Actinians. | Sedgwick’s theory was practically 
an alternative to Balfour’s, but | find myself able to give a 
partial acceptance to both these views. For the nervous 
system of Mollusca appears to me to consist of two parts, a 
circumferential ring and a peri-blastoporal plexus. The 
circumferential ring, which was primitively associated with 
a ciliated ring, is represented by the pleuro-visceral nervous 
system, which I have shown to possess significant relations 
with the velum or prototroch of the larva; and the peri- 
blastoporal plexus seems to me to be recognisable in the 
pedal nervous system, which in primitive Molluscs has a 
very diffuse plexus-like arrangement, and in Amphineura, 
at any rate, reveals its peri-blastoporal character in the 
