1 6 EYE. 



unpaired auditory and optic organs make their appearance, and 

 (2) a posterior nearly solid cephalic ganglion, through which 

 there passes a narrow continuation of the central canal of the 

 nervous system. This ganglion consists of a dorsal section 

 formed of distinct cells, and a ventral section formed of a 

 punctated material with nuclei. The auditory organ 1 consists 

 of a 'crista acustica ' (fig. 9), in the form of a slight prominence 

 of columnar cells on the ventral side of the anterior cerebral 

 vesicle ; to the summit of which a spherical otolith is attached 

 by fine hairs. In the crista is a cavity containing clear fluid. 

 The dorsal half of the otolith is pigmented : the ventral half is 

 without pigment. The crista is developed in situ, but the otolith 

 is formed from a single cell on the dorsal side of the cerebral 

 vesicle, which forms a projection into the cavity of the vesicle, 

 and then travels (in a manner not clearly made out) round the 

 right side of the vesicle till it comes to the crista ; to which it is 

 at first attached by a narrow pedicle. The fully developed eye 

 (figs. 8 VI. and 9, 0) consists of a cup-shaped retina, which forms 

 a prominence slightly on the right side of the posterior part of 

 the dorsal wall of the anterior cerebral vesicle, and of refractive 

 media. The retina is formed of columnar cells, the inner ends 

 of which are imbedded in pigment. The refractive media of the 

 eye are directed towards the cavity of the cerebral vesicle, and 

 consist of a biconvex lens and a meniscus. Half the lens is 

 imbedded in the cavity of the retina and surrounded by the 

 pigment, and the other half is turned toward a concavo-convex 

 meniscus which corresponds in position with the cornea. The 

 development of the meniscus and lens is unknown, but the 

 retina is formed (fig. 8 V. a) as an outgrowth of the wall of the 

 brain. At the inner ends of the cells of this outgrowth a deposit 

 of pigment appears. 



The trunk section of the spinal cord (fig. 9, N] is separated 

 by a sharp constriction from the brain. It is formed of a super- 

 ficial layer of longitudinal nervous fibres, and a central core of 

 ganglion cells. The layer of fibres diminishes in thickness 

 towards the tail, and finally ceases to be visible. Kupffer 

 detected three pairs of nerves passing off from the spinal cord to 



1 For a fuller account of the organs of sense vide the chapters on the eye and ear. 



