AUDITORY ORGAN'S — EUTHYNEURA AND PELECYPODA. 
241 
line, the otoconia of the Styloinmatophora being usually of a broadly 
oval form, while those of the Basonnnatophora are more lenticular 
Figs. 484. 485. 
Fig. 483. — Underside of the pedal ganglia of Hyaluiia alliaria (Miller), highly magnified, 
from Redcar, collected by Mr. Baker Hudson, showing the position of the otocysts upon them. 
ot. otocj^st ; c.c. cerebrO'pedal connective. 
Fig. 484 side view and Fig. 485 front view of otoconia from otocyst of Helix pomatia L., showing 
the central nucleus and radiate striation, highly magnified (after Leydig). 
in character. The otocystic nerve to each sac is a delicate fibril 
Ijdng between the cerebro-pedal and the cerebro-pleural connectives 
of their respective sides and arising from the cerebral ganglia. 
In Pelecypods, although the more archaic genera still retain the 
canal of invagination communicating with the exterior, yet in the 
adults of our forms the otocysts have become closed and contain only 
a single otolith in each sac, that of Sphwriim 
being a flattened sphere, with radial stria- 
tion, much smaller in size than the cyst 
contaiinng it, though other species not 
members of our fauna may possess a number 
of otoconia in each sac, or an otolith may 
be found with otoconia. In the fixed forms, 
as Dre/sseiisia, the otocysts are wanting in 
the adult stage, though present during development. 
The auditory nerves are closely joined with the cerebro-pleuro-pedal 
connectives, although their fibres arise from the cerebral centre. 
Fig. 486. — Otolith from the 
otocyst of Spha‘rium corncuvi 
(L.), highly magnified (after 
Simroth), showing its oval form, 
strong radial striation and 
peculiar nuclear space. 
Fig. 487. — Otocyst of left side of Anodonia^ showing its close connection with the pedal ganglia 
and the innervation from the cerebro-pleural ganglion, enlarged (after Simroth). 
aud.H. otocystic or auditor>* nerve ; c.p.g. cerebro-pleural ganglion ; c.pl.p.c. cerebro-pleuro-pedal 
connective ; ot. left otocyst, with otolith ; p.g. pedal ganglia. 
Uenerally speaking the auditory organs of the Unionida?- and the 
Ilelicida' form the ends of a series which the other groups unite by inter- 
mediate gradations. Some Streptoneures show by their otoliths more 
atfinity to the Pelecypods, while others approach the Styloinmatophora. 
1/2/97. 
