XII 



PHYLUM MOLLUSCA 



The nervous system is found in its most primitive condition 

 in Nucula (Fig. 601). Instead of the single cerebro-pleural ganglion 

 of Anodonta there are, on each side, distinct cerebral (XVI) and 

 pleural (I) ganglia, each united by a connective with the pedal. 



The most characteristic sense-organs are the statocysts 



(" otocysts ") and the osphradia. The statocyst " auditory " or 

 directive organ is always placed in the foot, in close relation to 



FIG. 602. Vertical section of eye of Pcctcn. 1, cornea ; 2, lens ; 3, external epithelium ; 

 4, blood-sinus 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and 

 Heider.) 



the pedal ganglion, sometimes embedded in the latter. The 

 statocysts are developed as involutions of the ectoderm and retain 

 their connection with the exterior in Nucula (Fig. 596, ot) and some 

 others. In most cases they become closed sacs. The cavity is 

 usually ciliated, but the cilia may be wanting. Each statocyst 

 may contain a number of minute statocones or, more usually, a 

 single, larger statoliih. The nerves supplying the statocysts are 

 given off not from the pedal ganglia, but from the cerebro-pedal 



