240 
AUDITORY ORGANS — STREPTONEURA AND EUTHYNEURA. 
these concretions, in one still younger Wagner observed only nine or 
ten, and Ponchet found in another example but seven, though the 
adult animal nsnally possesses more than a hnndred. The cysts also 
do not always contain an e(iual number of otoconia, nor are they 
exactly uniform in size and shape, even in the same sac. 
The otolith, which is the primitive form in which these concretions 
first arise during the embryonal develo})ment of all onr mollusks, is 
still retained by and more particularly characterizes those genera and 
species of Streptoneura and Pelecypoda, which in many other resi)ects 
are amongst onr most highly sjtecialized forms, whereas the more 
archaic genera of these groui)s and the Euthyneura lose the otolith 
jiresent during their embryonal life and ac([nire otoconia. 
'The otocysts are always more easilv examined in the immature 
than in the adult animal, even in the smaller species, as the whole 
body can be pressed between the glass slii)s and the action and 
movements of the otoconia studieil for tifteen minutes or more l)efore 
they tinally cease. In the Naiads this organ is much more diflieult 
of e.xamination and study. 
In the kStreptoneuro.s, the otocysts are not usually so intimately 
connected with the i)edal ganglia as in the Euthyneiires, and, as in 
the highly specialize I Cjjdo^tonut ekr/iiuft, may retain the primitive 
arrangement of a single large spherical otolith in each cyst, which 
almo.st blls the whole, vesicle, and in which 
the concentric lines of accretion or junction 
])redominate, although radial striation is 
also present, as when crushed it invariably 
breaks into live i)ieces. t'onie si)ecies which 
are, however, otherwise less highly specialized 
have an exceedingly large number of oto- 
conia, as Xf'rlfind iiri<(fiJi.-i ; those of 
Kig. 4S2. — Otolith from the 
otocyst of Cyclostoma elegans 
(Mull.), highly magnified (after 
Garnault), showing its spherical 
form, distinct concentric struc- 
ture and radiate striation. 
Vdh'dtd are quite like tho'C of the Helices, being oval and some- 
times angulated at the end, with a yellow central area and a darker 
nucleus, while certain forms halhtually ])ossess an otolith and 
otoconia in each cyst. 
In the Euthyneiires the otocysts are usually placed upon or 
actually imbedded in the pedal ganglia, the free side being usually 
more convex than the one attached to the ganglia. They contain 
numerous otoconia, which though generally more or less oval in shape, 
with minute central area, yet often show considerable variety of out- 
