THE LAMELLIBRANCHIA 237 
ganglion—and consequently on the posterior adductor muscle in 
most cases—there is an accessory ganglion (ig. 214, os), above which 
the tegumentary epithelium is modified to form a sensory organ, 
and is often pigmented, as for example in Arca. This organ cor- 
responds to the osphradium of the Gastropoda, and its situation at 
the point of attachment of the gill' renders it probable that it serves 
to test the respiratory fluid. The osphradial ganglion receives 
nerve-fibres not from the visceral ganglion, but from the cerebral 
ganglion by way of the visceral commissure. 
Another organ of an analogous nature, lying on the posterior 
adductor muscle on either side of the anus (and consequently 
behind the osphradia), is found in many asiphonate forms, for 
example, in the Arcidae, the Trigoniidae, the Pectinidae, and 
the Aviculidae. It forms a little papilla or epithelial elevation 
at the end of a nerve strand given off from the posterior pallial 
nerve, which in turn is given off from the visceral ganglion. In 
these Lamellibranchs the two little organs in question often show 
a tendency to asymmetry, that of the right side being better 
developed than that of the left. In the siphonate Lamelli- 
branchia, in which the gills are united together posteriorly 
and conceal the posterior adductor muscle, the sensory organ in 
question is displaced along the course of the posterior pallial nerve, 
and lies at the internal end of the inhalent or branchial siphon, 
often lying over a “siphonal” ganglion developed at this place. 
In such case the organ may be an epithelial projection in the form 
of a glandular and sensory plate (Leda, Donax, and Pholus), or a 
projecting lamina (Mactra, Scrobicularia, ete.), or even a tuft composed 
of many papillae (Tellina). Lastly, an adoral 
sense-organ, provided with an accessory ganglion, 
exists in the neighbourhood of the palps in Nucula 
and Poromya. 
The otocysts or statocysts, as is the case in 
the majority of the Mollusca, are situated in the 
pedal mass in proximity to the pedal ganglia 
(Figs. 230, of ; 242, ay), and they may even be —tocyst. of Cyclas. 
deeply embedded in these centres, for instance, % capsule; ¢, ciliated 
: : cells lining the otocys- 
in Galeomma, and the Leptonidae (Lasaea, Kellya). tic capsule ; 0, otolith. 
In the majority of the Protobranchia (Nucula, {eye er 
Leda, Solenomya), in Arca, and in some Mytilidae 
(Mytilus, Lithodomus) these organs are simply deep invaginations 
(otocrypts) of the superficial epithelium of the foot, and com- 
municate with the exterior by a fine canal which opens on the 
side of and in the dorsal region of the foot (Fig. 214, of, 0.0): 
this canal is closed in the adult Yoldia. In the adult Solenomya 
the otocysts have disappeared. In Leda they each contain an 
otolith, but in Nucula, Arca, and the Mytilidae they contain 
Fic. 216. 
