REPORT ON THE PTEROPODA. 
69 
We find in Natica that the disposition of the anterior suboesophageal ganglia resembles 
that seen in the Cephalopoda. In front of each pedal ganglion, in the position occupied 
by the brachial ganglion of the Cephalopoda, there is 
another corresponding suboesophageal ganglion, and this 
covers the head. I may here remark that the figures of 
the nervous system of Ncitica given by Souleyet , 1 which 
are the only original figures known to me, are inverted, 
that is to say that the upper (dorsal) surface is indicated 
as the lower (ventral) surface, and vice versa. It fol- 
lows from this that the peculiarity of the nervous system 
of Natica in possessing propedal ganglia has not hitherto 
been observed. 
The formation of these propedal ganglia and their 
separation from the pedal ganglia are evidently due to the great development of the 
anterior part of the foot, which has become transformed into a cephalic shield. Some- 
thing of the same kind must have taken place in the Cephalopoda, where the formation of 
the brachial ganglia has been brought about by the great development of that part of 
the foot which has entirely surrounded the head and produced the arms. 
It must not be concluded from what has just been said that I regard the shield of 
Natica and the arms of Cephalopods as exactly homologous ; I only wish to draw from 
these facts the following conclusions : — In Natica we observe the formation of a pair of 
propedal ganglia in consequence of great development of the anterior part of the foot ; 
in Cephalopoda we observe the same propedal ganglia ; we may conclude, therefore, that 
the organs which they innervate are a portion of the foot situated anteriorly, which has 
taken on considerable development. 
In Natica this anterior part of the foot covers the head by its anterior border, hence 
the coalescence with the head could not proceed further, because the mouth could 
not have remained open. In the Cephalopoda, on the other hand, it is the lateral 
margins of the foot which have invaded the head, leaving the buccal opening free ; the 
two halves have met on the dorsal aspect of the head, concrescence has taken place, and 
the head has thus become entirely surrounded by a pedal mass. 
Where, then, is the head ? asks von Jhering . 2 The postero-lateral portions of it are 
to be seen in Nautilus, with the eye, the olfactory groove, and the two tentacles ; and 
between the pedal appendages is seen the buccal mass. If the head be to a large extent 
concealed, it is not therefore non-existent. 
V. We have already seen how those naturalists who defend the views which regard 
1 Voyage de la Bonite, Zoologie, Mollusques, pi. xxxvi. figs. 13, 14. 
2 Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, p. 268. 
propedal ganglion innervates the voluminous mass wdiich 
Fig. 2. — The pedal ganglia of Natica. a, pedal 
ganglia ; b, propedal ganglia ; c, pedal com- 
missure. 
