_N´_. anterior swelling of neural tube; _N._ anterior swelling of spinal portion of neural tube; _n._ hinder part of neural tube; _ch._ notochord; _K._ branchial region of alimentary tract; _d._ oesophageal and gastric region of alimentary tract; _O._ eye; _a._ otolith; _o._ mouth; _s._ papilla for attachment.] The original single vesicle of the brain becomes divided by the time the larva is hatched into two sections (fig. 9)--(1) an anterior vesicle with, for the most part, thin walls, in which 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[8] 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, _O_) 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. [8] For a fuller account of the organs of sense _vide_ the chapters on the eye and ear.