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POPULAR SCIENCE REVIEW. 
organ of hearing ; while from the parieto-splanchnic ganglion of 
either side runs, posteriorly, a branch to the rudimentary 
siphon, anteriorly, a cord to the gills, and, laterally, a branch 
to the hinder region of the mantle-lobe. In Lamellibranchs 
with well-developed siphons — e.g. Pholas — accessory siphonal 
ganglia are present, which are joined by a commissure to either 
half of the bifid branchial ganglion. 
Although the Lamellibranchs, including the river-mussel, 
are well endowed with general sensibility, only one organ of 
“ special sense ” has yet been discovered in them, namely, that 
of hearing. It consists of a globular sac lined with ciliated 
epithelium, and containing a fluid in which floats a single 
round otolith * composed of carbonate of lime. The vibrations 
of this otolith, which are very easily excited, are transmitted 
to a nerve sent to the sac from the pedal ganglion. All organs 
of hearing, even the most' specialised, such as that of the 
highest vertebrata, seem to require as one of their constituents, 
a sac containing otoliths floating in fluid ; such is represented 
in ourselves by the “ utricle ” of the membranous labyrinth of 
the internal ear, where hexagonal crystals of carbonate of lime 
may be found floating in the “ endolymph.” 
Although no special organ of sight has yet been discovered 
in Anoclonta like unto the eyes which, “ smaragdino colore 
coruscantes,” as Poli says, stud the edge of the mantle in the 
scallop, this animal is undoubtedly sensitive to light. An 
American naturalist, from observations carried on in the rivers 
of central Iowa, states that Unios take cognisance, by sud- 
denly closing their valves, of both admission and cutting off of 
light ; and that it is, moreover, to the light rays that they are 
sensitive, for the same action will take place after the heat 
rays have been previously intercepted. 
It is not improbable, though it yet remains to be proved, that 
the appendages which fringe the orifice of the rudimentary 
inhalant siphon (fig. 1, s) of Anodonta may each turn out to be 
a simple form of eye ; this being supported by the fact that in 
Pecten the whole rim of the mantle is brought into view by the 
gaping of the shell, while in the river-mussel it is only the 
hinder margin of this organ which is ordinarily exposed and 
upturned to light. 
A few remarks on the homologies of Lamellibranchs may not 
be amiss. 
The shell of Anodonta is homologous with the c< test” of the 
• Anglic^ — “ear-stone,” from ouc,wroc,=ear, and AifJoc=stone. When only 
one auditory sac is developed, as is sometimes the case, it is said to contain, 
by way of compensation, as it were, two otoliths. In the Gasteropoda the 
otoliths are numerous, but are crowded together in one central mass. 
