198 THE ACEPHALA. $ 183. 
A. The central nervous mass is composed of three pairs of principal 
ganglia, as follows: Par anterius or labiale, Par posterius, and Par 
inferius or abdominale. This last pair is extraordinarily developed with 
those species which have a foot, and has, therefore, received also the name 
of Par pedale. ' 
The ganglia of the Par anterius are situated one on each side of the di- 
gestive canal, and are connected by a filament which extends arcuately 
over the circumference of the oral cavity. They send off two long 
nerves, which pass along the back to the Par posterius, which is the larg~ 
est of all and usually situated upon the anterior surface of the posterior 
adductor muscle. Its two ganglia are either blended together, or connected: 
by a transverse commissure. In this way, the anterior and posterior 
pairs with their commissures form a kind of cesophageal ring which sur- 
rounds the base of the abdomen. : 
The inferior or pedal pair is situated at the point where the base of the 
foot is joined to the abdomen. Its ganglia are contiguous upon the median 
line, or are blended together into one. They connect also with the labial 
ganglia by two nerves, thus forming a second cesophageal ring. 
Besides these principal ganglia, there are others, smaller, and situated in 
various parts of the body. But these are not constant, for their presence is 
always due to an unusual development of the muscular system. 
B. The peripheric nerves arise almost exclusively from the three pair of 
principal ganglia, for the nerves of connection do not. usually give off 
branches. The few and very delicate filaments which sometimes pass off 
from these last, belong, probably, to the splanchnic system, for the. princi- 
pal ganglia appear to furnish only sensitive and motory nerves; these are ~ 
usually distributed in the following manner : 
The Par anterius sends nerves to the anterior part of the mantle, to 
the anterior adductor muscle, and to the tentacles of the mouth and its cir- 
cumference. : 
The Par posterius sends two very large trunks to the branchiae; also 
other nerves to the lateral and posterior part of the mantle, to the pos- 
terior adductor muscle, and delicate filaments to the heart and rectum. 
5 The length of the arc of this filament of con- Jer’s Arch. 1840, p. 33, Taf. IIL. fig. 3, g.; and * 
nection depends upon the position of the two labial Blanchard, loc. cit. p. 336, Pl. XII. fig. 3, a. b.), 
ganglia. Thus with Pecten, where these ganglia between these ganglia with which it is in connec- 
are situated unusually in the rear, itis very long tion by commissures. 
and very arched; while with Pholas, and Solen, 8 Of these three ganglia, the Par anterius has. 
where they are close upon the oral opening, it is often been regarded as the brain. But others have 
short. With Venus, and Mactra, these ganglialie rather taken the Par posterius for the principal 
80 close to each other, that this anastomotic fila- nervous mass. For my own part I think that all. 
ment is replaced by a very short, transverse com- three, together with their commissures, correspond 
Inissure. : to the pharyngeal system of Gasteropoda. 
6 The Par posterius, which, from its relations 9The anterior nerves of the mantle of Solen, 
to the branchiae, is also called the Par branchiale, which is prolonged far beyond the oral opening, 
is blended into a single ganglion in those species and is strengthened by a muscular mass— have 
whose branchiae are united at their lower part; as ten to twelve ganglia lying along the border of the 
Unio, Anodonta, Mactra, Mya, Solen, and Pho- mantle. With Pecten, the mantle-nerve has also 
das. On the other hand these ganglia are separate, a small ganglion upon the muscular mass which is 
and connected simply by a transverse commissure, found upon each side of the anterior border of this 
with those whose branchiae are isolated; as Os- organ; see Blanchard, loc. cit. p. 333, Pl. XII. 
trea, Pecten, Avicula, Mytilus, Lithodomus, fig. 1, f. (Solen), fig. 3, c. (Pecten). 
Modiola, and Arca. : 10 In the mantle of Ostrea, Spondylus, Pecten, 
7 According to the earlier Zootomists, the Par Lima, and in general those species in which its 
pedale was wanting in those species which have borders have numerous sensitive organs, the 
no foot, although careful investigation has shown branches of the anterior and posterior mantle- 
that there is a pair corresponding to the Par infe- nerves unite and form a common marginal nerve ' 
rius. I refer to that found with Ostrea (Brandt, whose size depends upon the number of the sensi-- 
loc. cit. Tab. XXXVI. fig. 11, a. 0.), directly behind tive organs to which it sends filaments. 
the labial ganglia, and with Pecten (Grube, Mil- 
