The ORGANS OF SENSE in _Nereis_ are eyes, tentacles, palps, and cirri. The four eyes, which rest upon the brain, have the structure represented in Fig. 130. The retina consists of a single layer of cells containing pigment; each cell is drawn out peripherally into a nerve-fibre, whilst centrally it forms a cuticular product—the "rod" (_h_). The edges of the retina are continuous with the surrounding epidermis, and the cup thus formed remains widely opened to the cuticle in a few Polychaetes, _e.g._ _Autolytus_, and in the young of _Nereis_, but more usually it has the relations represented in the figure. The lens is produced by the retinal cells (according to Andrews[310]), and is in some cases (_Eunice_, _Amphinome_) continuous with the cuticle. It appears to be composed in other cases (_Lepidonotus_) of continuations of the retinal rods. The structure of the other sense organs {256}indicates their adaptation to a tactile function; in each case a nerve traverses the axis of the organ, and the nerve-fibrils terminate in sensory cells. Very probably the palps have a certain power of testing the food—a combination of the senses of taste and smell. The GENERATIVE SYSTEM.—In all the Polychaeta, with very few exceptions, the sexes are separate; and the reproductive cells—ova and spermatozoa—are produced at certain seasons of the year by the rapid proliferation and modification of coelomic epithelial cells surrounding the blood-vessels in the parapodium and its immediate neighbourhood. The sexual cells remain in the coelom till they are ripe. The egg-cells become filled with yolk globules; a vitelline membrane is present, and an outer coat of albuminous material. It is doubtful by what means these sexual cells are discharged in _Nereis_. There is some evidence that the "dorsal ciliated organ" may act as a genital duct. In some other worms the nephridia serve this purpose, whilst in others a rupture of the body-wall allows the products to escape into the sea. According to Wistinghausen,[311] at the time of discharge the females of _Nereis dumerilii_ become surrounded by a kind of gelatinous tube formed from a secretion of the parapodial glands, and into this tube the ova are discharged, and arranged in a single layer round its wall. The common species _Nereis diversicolor_ is viviparous. In a large number of species of _Nereis_ the sexually-mature individuals undergo very marked changes in various parts of their body, so that they differ very greatly from the immature individuals. These changes resulting in the "heteronereid" condition will be dealt with at some length in Chap. X. p. 276. The larvae of Polychaetes and other facts connected with reproduction are described in the same chapter. {257}CHAPTER X CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE— GILLS—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS— BUDDING—FISSION—BRANCHING—REGENERATION.