ANATOMY OF A FRESH-WATER MUSSEL. 



55 



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

 splanchnic commissures through the substance of the nephridium 



Fig. 21. Diagram of the nervous 

 System of a Mussel. 



Fig. 22. a, Gauglionic or nerve 

 cell with nucleus ; b, nerve-fibre 

 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 Sphtzrium 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 Pisidium and Spharium, 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 



