36 Moll 
MOLLUSCA. 
proboscis, the outer giving off a slender nerve, at once tentacular and 
optic. Between these bundles are two formed by the pedal nerves. The 
light viscoral commissure bifurcates almost immediately, and sends a 
strong branch into the neighbouring tissues ; the visceral loop is long and 
narrow. In Pleurotoma babylonius the nervous system resembles that of 
Terebra in all essentials, whilst in P. nodifera it is on the same type as 
that of Conus virgo. It appears that the Toxoglossa are united by 
characters mainly negative ; they may be arranged in a series leading 
up from Buccinum, and perhaps ending in the Cuncellariidce, whilst the 
Prosobranchiata , as a whole, have a constancy in the structure of the 
nervous system, which justifies their erection into an order by Milne- 
Edwards ; within this group may be clearly distinguished two sub- 
divisions— one with two and the other with three nervous collars ; 
Bouvier (5). 
The Neritidcc and Helicinidcs present the three characters common to all 
Scutibranchiata : — (1) Four anterior pericesophageal nervous collars; (2) 
the origin of the stomatogastric system in a gangliouic proboscidian pro- 
cess on the cerebral ganglia ; (3) the existence of two long, pedal, more or 
less ganglionic cords, usually connected by transverse anastomoses. They 
are distinguished, however, by the orthoneurous pallio-visceral system, a 
character peculiar to these two families among the Prosobranchiata. 
This and other modifications are perhaps correlated with a change from 
inarine to fresh- water and terrestrial life, and may be illustrated in a 
progressive series in the following forms : Nerita peloronta , Neritina 
canalis, Navicella , Helicina brasiliensis, and H. sagraiana ; Bouvier (7). 
Ampullaria is both chiastoneurous and zygoneurous ; the right pallial 
nerve supplies the penis, which is an appendage of the mantle (apparently 
a unique arrangement). The epipodium is of pallial origin, being inner- 
vated by the commissural ganglia. The gills are homologous with the 
left gill and osphradium of the Zeugobranchiata. The genus should be 
placed among the zygoneurous Taniioglossa near the Caly ptncidcc ; 
Bouvier (2). 
Under Scutibranchiata are arranged a number of forms of Cyclobranchi- 
ata and Aspidobranchiata : they have (1) a loug cerebroid commissure, 
the ganglia having a forward projection united with that of the opposite 
side by a suboesophageal commissure ; (2) the stomato-gastrio system 
arising from this projection and the sympathetic ganglia widely separated ; 
(3) well-devefoped pedal ganglia and cords ; (4) pallial ganglia united 
with the pedal. These characters are of primordial significance ; 
Bouvier (1). See also id., (6) pp. 29 & 30 antea. 
In Dolabella rumphii the cerebral ganglia are united by a short com- 
missure and give off six pairs of nerves ; the most important are the ten- 
tacular, optic, and acoustic. The cerebro-pedal connective is thick, the 
ccrebro-visceral thin. Besides connecting cords, the visceral ganglia give 
off two pairs of nerves distributed to the integuments and the mantle. 
There are twelve or thirteen pairs of pedal nerves. The posterior 
visceral nerves are close together to the right of the middle line, and 
give off branchial and genital nerves. The distribution of the stomato- 
gaslric system corresponds with the disposition of the digestive tract, 
