91 



the surface of the ))ody, and somewhat between the 

 ganglia. 



They can be seen in dissections as two minute delicate 

 white bodies, if the connective tissue above the cerebral 

 and pedal ganglia is carefully removed, and are perfectly 

 spherical in shape, wnth a diameter of 0'17 mm. 



The position of the otocysts outside the foot is 

 interesting, as the pedal ganglia are also completely 

 outside the foot in Pecten. There can, however, be no 

 question here of the nerve innervation, for the nerves can 

 be traced directly out from the cerebral ganglia without 

 any connection with the cerebro-pedal connectives, thus 

 rendering further proof to the theory that the otocysts 

 are alw-ays innervated by the cerebral ganglia, even when 

 they are connected with the cerebro-pedal connective and 

 lie in close proximity to the pedal ganglia. The nerves 

 enter the otocysts on the sides facing the cerebral ganglia, 

 and are thus quite short. The otocysts in both P. 

 opercularix and P. maximus are spherical closed sacs, the 

 internal cavity of which is bounded by a layer of sense 

 cells. In this cavity there is situated an otolith. This 

 appears in P. opercuLatis as a large ball, formed of small 

 crystals of irregular shape, as if a heap of fine detritus 

 had been heaped together. The otolith thus formed fills 

 almost completely the cavity of the otocyst. In 

 P. maximum, I have only found some larger and more 

 scattered crystals (otoconia) in the otocyst. In the adults 

 of both species examined, the otocyst has no duct 

 connecting the cavity with the external world. 



TiiE OsPHRADiA. — These paired sensory structures, 

 named by their discoverer, Spengel (38), organs of smell, 

 aud which are of general occurrence in Mollusca, are not 

 highly developed in Pecten. They cannot be detected 

 without the aid of microscopic sections, and though 



