240 TRANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



On the inner portion of the retina is poised a gelatinous 

 non-cellular lens, whose outer margin, facing the cornea, is 

 smooth and evenly convex, but whose inner edge, abutting on 

 the retina, is ill-defined. Hair-like processes, known as 

 " Stiftchen," project from the retina to hold the lens in place. 



Blood lacunae surround the eye, and blood is brought to 

 it by a branch of the cephalic artery. (Fig. 13, opt. a.) It 

 is innervated by a small optic nerve (Fig. 16, opt. n.) which 

 arises from a comparatively large optic ganglion attached to 

 the cerebral ganglion. 



The Statocyst. 



The statocysts or otocysts (Fig. 23, ot.) are situated on 

 the anterior face of the pedal ganglia. Each otocyst is a 

 hemispherical sac whose flattened base is attached to the pedal 

 ganglion of its side. The wall of the sac consists of a thin 

 non-ciliated epithelium. The connective tissue which enwraps 

 the whole nervous system covers the otocyst too, and separates 

 it from the nervous tissue of the pedal ganglion. Within the 

 otocyst is a granule of calcareous matter. The sac is innervated 

 from the cerebral ganglion, the auditory nerve lying between 

 the cerebro-pedal and cerebro-pleural connectives. The otocyst 

 forms the organ of orientation, and like the eye is epithelial 

 in origin. 



The Osphradium. 

 (Olfactory Organ, Organ of Spengel.) 



The single osphradium is situated at the base of the 

 ctenidium on its anterior face. In the living condition it can 

 usually be recognised as a yellowish patch, due to the branchial 

 ganglion showing through the skin at this point and not to the 

 osphradium itself. The osphradium is the water-testing organ. 

 Should the water approaching the ctenidium become deficient 

 in oxygen, the nerve impulse generated by the osphradium 

 causes the animal to open its parapodia and thrust out the 



