256 POLYCHAETA 



cates 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 epi- 

 thelial 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, 1 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. 

 1 Wistinghausen, " Entwick. v. N. dumerilii" ML Zool. Stat. Neapel, x. 1891, p. 41. 



