PHYLUM MOLLUSOA 



707 



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 

 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. 586 ot) 

 and some others. In most cases they 

 lu'come 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 statolith. The nerves sup- 

 plying the statocysts are given off not from 

 the pedal ganglia, but from the cerebro- 

 pleural connectives, and their fibres are 

 derived from the cerebral ganglia. 



The osphradia — " olfactory " or water- 

 testing organs — are patches of sensory 

 epithelium with an accessory or osphra- 

 dial ganglion situated in the immediate 

 relation with the visceral ganglia (Fig. 

 591, viii), but connected by nerve-fibres 

 with the cerebral ganglion. Patches of 

 sensory epithelium, very similar to the 

 osphradia, and called the ahdominal sense- 

 organs, occur one on each side of the 

 anus in Area and other forms devoid of 

 siphons, and a similar organ has been 

 described beside the retractor muscles of 

 the siphons in several Sinupalliata. 



In a few instances eyes are present, 

 but never in what we are accustomed to 

 consider as the normal position for such 

 organs, at the anterior or head-end of the 

 body. They occur, in fact, in the only 

 situation where they can be of any use, 

 namely, along the edge of the mantle. 

 The best-known form in which they occur 

 is the common Scallop {Pecten), which has 

 a single row (Fig. 575, VII) all round the 

 mantle-border. Each has a cornea (Fig. 592 1), a cellular (not 

 cuticular) lens (;2), a retina (5) — formed of cells, the inner ends 

 of which are modified into visual rods, and an optic nerve (7), 

 one branch of which spreads over the front of the retina and 

 sends branches backwards to the visual rods. In this peculiarity, 

 as well as in the cellular lens, the eye of fecten is singularly 



Fig. 591. — Nervous syatem and 

 "auditory" organs of Nu- 

 cula. I, pleural ganglion ; 

 [I, pleuropedal-connective ; 

 III, common connective from 

 cerebral and pleural to pedal 

 ganglia; IV, "auditory'* 

 nerve ; V, pedal ganglion ; 

 VI, visceral ganglion ; VII, 

 posterior pallial nerve ; VIII, 

 osphradial ganglion ; IX, 

 visceral connective ; X, stato- 

 cyst ; XI, canal of statocyst ; 



XII, its external aperture ; 



XIII, cerebro-pedal connec- 

 tive ; XIV, anterior pallial 

 nerve ; XV, nerve to palps ; 

 XVI, cerebral ganglion. (From 

 Felseneer.) 



