REPRODUCTION AND GROWTH 265 



considerable difficulty to those working out the different 

 life-histories. 



Among Polychaets the forms known as " Palolo worms," 

 which live in crevices of rock or of coral reef, are remarkable 

 for their habit of swarming at the period of reproduction. 

 The first of these worms to attract attention was the Pacific 

 species Eunice viridis, but others with the same habit are 

 the Atlantic Palolo {Eunice fucata) and the Japanese form 

 {Nereis japonica). These forms agree in showing at the time 

 of swarming two distinct body regions, one of which con- 

 tains the reproductive elements. In each species the 

 swarming takes place every year with the utmost regularity 

 in the same month or months, at the same particular phase 

 of the moon. During the swarming the sexual portion 

 breaks off and carries away the sex products. So extra- 

 ordinarily numerous are the worms constituting the swarms 

 that to a Japanese observer they appeared to cover the 

 whole surface " as with a sheet " (Akira Izuka, 1908 ; and 

 for the breeding habits of the Atlantic Palolo, see Mayer, 

 1908). 



Several species of Polychaets deposit ova surrounded by 

 a mass of jelly. Such cocoons, which are demersal, are 

 frequently encountered in rock pools between tide-marks, 

 and yet considerable doubt still exists as to the precise 

 species to which they belong. The eggs of Phyllodoce 

 are green in colour and are enveloped in a spherical 

 mass of gelatinous material secreted by the skin of the 

 worm. M'Intosh (1908) has observed them in May, and 

 further states that the ripe male is distinguished by the pale 

 yellow hue of the body. Ashworth (1904) states with regard 

 to the lug- worm that " in spite of the abundance in innumer- 

 able places of the adult worm, and of much searching by 

 many workers, the egg masses of Arenicola have never been 

 found on the coast of Europe." Various writers, however, 

 have brought forward a considerable amount of circum- 

 stantial evidence as to the relation of certain globular masses 

 of green spawn to Arenicola. Hornell (1891) has found 

 large, green, pear-shaped egg-masses associated with the 



