THE LAMELL1BRANCHIA 237 



ganglion and consequently on the posterior adductor muscle in 

 most cases there is an accessory ganglion (Fig. 214, os\ above which 

 the tegumentary epithelium is modified to form a sensory organ, 

 and is often pigmented, as for example in Area. This organ cor- 

 responds to the osphmdium 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 asiphoriate 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 Pholax), or a 

 projecting lamina (Mactra, Scrobicularia, etc.), or even a tuft composed 

 of many papillae (Tellinu). 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 pio 216 



(Figs. 230, ot; 242, ay), and they may even be otoc ^ t of '. 

 deeply embedded in these centres, for instance, c > capsule; e, ciliated 



< , ,, T i / r 77- T/ \ cells li'iiiitf the otocys- 



in Galeomma, and the Leptomdae (Lasaea, Kellya). tic capsule ; <>, otoiith. 

 In the majority of the Protobranchia (Nucula, ^ijf******' *** 

 Leda, Solenomya), in Area, and in some Mytilidae 

 (Mf/tilus, 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, ot, o.o) : 

 this canal is closed in the adult Yoldia. In the adult Solenomya 

 the otocysts have disappeared. In Leda they each contain an 

 otoiith, but in Nucula, Area, and the Mytilidae they contain 



