74 The Swan-Mussel and its Anatomy. 



cipal ganglia — a cerebral pair at the sides of the mouth, a 

 pedal pair in the foot, and a third pair on the under surface of 

 the posterior adductor muscle, which are commonly called 

 ' branchial/ but which, as they supply not only branchial, but 

 visceral and pallid filaments, may more properly be termed 

 1 parieto-splanchnic' Three sets of commissural filaments 

 connect the cerebral ganglia with one another, with the pedal, 

 and with the parieto-splanchnic ganglia. The inter- cerebral 

 commissures surround the mouth, and the other two pairs of 

 cords extend respectively from the cerebral to the pedal, 

 and from the cerebral to the parieto-splanchnic ganglia."* 



With regard to special organs of sense, the fresh-water 

 mussels appear to possess only those of touch and hearing. 

 The terminal portion of the foot is said to be most suscep- 

 tible of touch, which sense may reside also in a slight degree 

 in other parts of the body, such as the mantle and labial palpi. 

 The organ of hearing, according to Siebold, consists of two 

 round sacs containing fluid and an otolith each; they are 

 situated on a pair of the nerves which spring from the pedal 

 ganglia. The reader may readily observe the form and posi- 

 tion of the auditory organs in the very common Cyclas ; if the 

 animal is removed from the shell and the foot be pressed 

 between two pieces of glass and viewed under the microscope, 

 the observer will see at its upper part two circular sacs with 

 their contained otoliths lying on the nerves of the pedal 

 ganglia ; the otoliths keep constantly oscillating within the 

 sacs. 



"• Aquatic molluscs, especially the acephalous kinds/' M. 

 Moquin-Tandon observes, " appear more sensitive of sound 

 than the others (terrestrial). A very loud noise is necessary 

 to cause a Helix to take refuge in its shell, or even to contract 

 its tentacles. The Limncei and the Planorbes appear a little less 

 indifferent, but the Cyclades and JJnios retract their syphon or 

 their foot on the slightest disturbance of the air or water, 

 even when one raises the voice or whistles near the vessel 

 which contains them." " I have often remarked," says Bau- 

 don, ' e that the Anodons drew in their foot and confined them- 

 selves obstinately within their shell, when one spoke loudly or 

 made any noise, and that without agitation of the water. One 

 day when I had put Anodonta cygnea (var. Celensis) on a 

 shelf in my room (they were out of water), I opened the door 

 rather noisily and I saw my Anodons, whose feet were much 

 elongated and were feeling the wood in every direction, I saw 

 them, I say, draw in their feet at the sound of the door. I 

 wished to repeat the experiment, and this time I opened tho 

 door softly with very little noise ; then again the Anodons drew 

 * Huxley's Elements of Comparative Anatomy, p. 35. 



