SCHARFF : ORGANS OF SENSE IN MOLLUSCA. 309 



object on account of the simple construction and the large size 

 of its octocyts. 



The ear capsule is enclosed within a more or less homo- 

 geneous membrane ; internally it is coated by the nervous layer, 

 which is again followed by the cellular layer (fig. 2). The 

 latter surrounds the cavity of the ear, which contains a fluid 

 and one or more calcareous granules. These are the ' otoliths' 

 or ear-stones, also present in the vertebrate ear. The otoliths 

 are moved about in the fluid by the cilia with which the cells 

 of the innermost layer are provided. The condition which I 

 have just described is that presented by Cyclas. In the Unio 

 group the cellular layer is much thicker. That this layer is 

 ciliated is probable, but has not, to my knowledge, been 

 definitely established. The general structure appears to be 

 pretty much the same in Gasteropods and Lamellibranchs, 

 excluding the Unionidse, which are distinguished by their 

 possessing a spongy capsule. The main differences in the ear 

 of the two groups lie in the nature of the cellular layer. In 

 Planorbis, Ancylus, and Limnaea the component cells of this 

 layer are very large and few in number. Neritina has cylindrical, 

 and Paludina as well as most Gasteropods, has cubical cells. 



With regard to the otoliths, I have mentioned that they 

 are of a calcareous nature. Their numbers are very variable. 

 Paludina vivipara has only one large ear-stone. Other shells 

 have several, and their number in some forms is enormous. 



A. Schmidt has discovered a canal in Helix, Limax, and 

 Physa leading from the auditory organ to the exterior. A 

 similar canal has also been demonstrated in Succinea and 

 Neritina, by Ball, who believes it is characteristic of all Gastero- 

 pods. Simroth seems to doubt the existence of such a structure. 

 He is of opinion that the canals have been produced by the 

 protrusion of the otoliths. The slender framework of the ear 

 subjected to pressure under the cover-glass might have caused 

 their formation. 



