ANATOMY OF A FRESH-WATER MUSSEL. 



55 



From the cerebral ganglia, two cords also proceed as the cerehro- 

 splanchnic commissures through the substance of the nephridium 



X|* N. 



Fig. 21. — Diagram of the nervous 

 System of a Mussel. 



Fig. 22. — rt, Ganglionic or nerve 

 cell with nucleus ; /', nerve-Ilbrc 

 showing contained granules. 



to two yellow parieto-splanchnic ganglia on the underside of the 

 posterior adductor muscle. These ganglia send branches to that 

 muscle, and to the foot. 



The Organs of Special Sense. — Many years ago Siebold 

 discovered in the foot of Sphcsmwi corneum a " small sacculus," 

 which contained " a cretaceous nucleus of a crystalline structure 

 performing remarkable oscillatory movements," and this he very 

 properly regarded " as a rudimental organ of hearing." In 

 Anodonta this auditory organ or otocyst is situated near the pedal 

 ganglia, and it has its nerve from the cerebro-pedal commissure. 

 Lankester thinks that an organ described by him as the osphradium, 

 consisting of a layer of .elongated epithelial cells in relation with 

 the osphradial ganglion, subservient to the sense of smell, while 

 the osphradial ganglion itself is considered by some as a co- 

 ordinating centre for sensations received from the inhalent current. 

 The labial palps may be organs of taste ; the general surface of 

 the animal is sensitive to touch. 



The Reproductive Organs.— With the exception of a few, 

 such as Pisidiiim and Sphcsrium, all the Lamellibranchs are 

 dioecious, a condition of things which was first made known by 

 Leuwenhock. Anodonta, however, is sometimes found to be 

 monoecious. The generative gland is a racemose gland situated 

 in the visceral mass. The ovary cannot well be distinguished 

 from the testis, except upon a microscopical examination, although 



