Hogg, on the Water-Snail. 
101 
and from the sub-oesopliagial ganglion, which fully equal the former in 
size, arise those nerves which supply the muscles of the body, and of the 
viscera." * 
The singular adaptation of the eye must not be omitted ; 
this appears in the early embryonic stage to be situated within 
or on the tentacle, it is constantly retracted with it, which is 
due to the length of the pedicle, and to the retractile sheath 
of the optic nerve, enabling the animal to shorten it ; at the 
same time the tentacle folds down over it, forming a protec- 
tive cover at all times. The eyes are situated at the base of 
the inner side of the tentacle, and resemble two very small 
black spots. When examined with a power of 100 diameters, 
they are seen to be transparent spherical lenses, surrounded 
by a black zone or iris, the pigmental layer is continued some 
distance down the pedicle. It is pear-shaped, and evidently 
the little animal is very quick-sighted, as he avoids every 
obstacle placed in his way, or quickly withdraws himself into 
his house if one attempts to touch him ; although in avoiding 
obstacles he appears to make great use of his tentacles as true 
feelers. The tentacles are composed of a dense elastic tissue, 
surrounded by a band of muscular fibre ; in shape they are 
triangular, with the base attached to the body of the animal. 
The Limnei are stated by Professor Forbes to have been 
found in the fossil state as far back as the Oolitic epoch ; and 
the most ancient forms bear a striking resemblance to the 
common existing types. In England, at the present time, they 
are abundant in nearly all the waters where vegetable matter 
is growing, and in the slow running rivers, especially where 
the water-cress is found. 
The Limneus, like every other living thing, is infested with 
its parasite. Reaumur observed a sort of mite infesting the 
snail {Helix aspersa)^ they were securely lodged in the pul- 
monary cavity. Miiller also noticed in certain Gasteropods a 
worm; and Dr. Gould, examining a specimen of the Physa 
heterostropha, " found the neck of the animal beset with 
numerous little things, looking like short, minute, white lines, 
attached like leeches, and which derive their nourishment 
from the fluids of the animal without his having the power to 
dislodge them." 
M. Baer states that he discovered a Filaria in the abdomen 
of Limneus stagnalis ; and in many of the same family of 
Mollusca he has met with a worm allied to the Naides, 
" living in the respiratory cavity, or hanging like little tufts 
of threads from the sides of the abdomen ; whence he named 
it ClicelogasterJ' Besides these, he says, " a kind of Cercaria 
* Professor Rymer Jones, op. cii. 
