100 
Hogg, on the Water-Snail. 
supplied with valves. In its course it takes a considerable 
turn around the inner whorls of the shell, terminating in a 
rectum which has its vent placed between a small portion 
of the mantle and the under edge of the last whorl of the shell. 
The liver is not nearly so large as it is in the land-snail ; it 
consists of two lobes, and is enclosed in a strong capsular 
covering ; it pours a pale-coloured bile into the stomach by 
more than one duct, and is provided with a proper hepatic 
system of vessels. 
The heart is a strong muscular apparatus, having both an 
auricular and ventricular cavity ; it is surrounded by a very 
delicate membrane (the pericardium). In shape it is pyriform, 
with muscular cords stretching from side to side, of a highly 
elastic character, looking not unlike very fine bands of India- 
rubber alternately contracting and expanding ; these cords are, 
no doubt, analogous to the cordce tendinece of the mammal 
heart. The heart receives the aerated blood from the respira- 
tory organs, and propels it through the vessels at the rate of 
sixty times a minute. It is placed far back in the superior 
portion of the shell, near to the axis, where it is securely fixed 
without reference to the movements of the mouth or body of 
the animal. 
Like others of this family of aquatic Gasteropoda, the 
breathing apparatus resembles the branchiae of fishes in struc- 
ture ; they are pectinated, and placed in three or four rows 
near the roof of a cavity under the integuments of the head, 
or rather above the oral opening, which is peculiarly arranged 
with retractor and other muscles, for the purpose of permitting 
an uninterrupted aeration of the blood as it is brought to the 
branchiae. 
The nervous system consists of many ganglise, or nervous 
centres, in place of a distinct brain, but " each of these 
gangliae may be considered as a distinct brain of the hetero- 
gangliate form." They are freely distributed throughout the 
body, but connected with each other by cords of communica- 
tion ; the nervous mass appears to be granular, and is some- 
what yellow in colour, whilst the nerves themselves are white 
and smooth, and invested with a delicate membrane {neuri- 
lemma). Professor Jones observes that — 
" One remarkable circumstance may be mentioned as peculiar to this 
class ; the changes of position of the nervous centres obey the movements 
of the mouth, with which they are intimately connected ; they are, in 
fact, pulled backwards and forwards by the muscles serving for the pro- 
trusion and retraction of the oral apparatus, and are thus constantly 
changing their relations with the surrounding parts. 
" The ganglia, placed above the cesophagus, sends off branches to supply 
the muscles of tlie head, the tentacles, and give origin to the optic nerves ; 
