MOLLUSCA. 121 



inhalent aperture are well supplied with fibres from the visceral 

 ganglia, and appear to be specially sensitive. The palps, tentacles, 

 and mantle margin are provided with tactile cells, each of which 

 is elongated and provided at its external end with a bunch of 

 delicate setse. 



(2) Gustatory Organs. The labial palps (?) possibly subserve 

 the function of taste. 



(3) Olfactory Organs. Each of the branchial nerves is covered 

 at its beginning by a patch of sensory epithelium known as an 

 osphradium, and usually considered as a sort of olfactory organ 

 which perhaps tests the quality of the water entering the pallial 

 chamber. 



(4) Auditory Organs. Two minute vesicles, the otocysts, are 

 found just behind the pedal ganglia, each of them being placed 

 at the end of an auditory nerve which comes off from the lower 

 end of the corresponding cerebro-pleural connective, and the 

 fibres of which probably run up the connective to the cerebro- 

 pleural ganglion of the same side. 



The otocysts are lined by ciliated columnar epithelium of 

 sensory nature, and each of them contains a rounded otolith 

 made up of concentric layers of carbonate of lime. 



The otocysts are difficult to study in Anodon, but are readily seen under 

 the microscope in a living specimen of Cyclas, a small freshwater bivalve 

 abundant in many streams and canals. 



The mussel never at any time possesses eyes. 



LIFE-HISTORY. 



The ova are fertilized and the oosperms developed, up to a 

 certain point, in the water-tubes of the outer gills of the female, 

 the interlamellar junctions of which secrete a nutritive substance. 

 It is an interesting fact that the shell commences as a single 

 saddle-shaped plate on the dorsal surface. As the mantle-lobes 

 are formed^this is divided into the two valves, which remain 

 connected by the ligament, so that this must be regarded as an 

 uncalcified part of the shell. Each valve is triangular, and its 

 lower pointed end is bent inwards into a sharp spine. In the 

 hinder part of the body a pit is formed, the lyssus gland, which 

 secretes a long sticky filament or lyssus. 



