264 THE BIOLOGY OF THE SEA-SHORE 



of tubicolous worms are frequently attached to the tube, 

 either inside or outside — a distinct advantage. According 

 to Leschke (1903), the eggs of Terehella zostericola and of 

 Terehellides strcemi are laid in a mucous mass attached to 

 seaweed or to the mouth of the tube. In Spirorbis the 

 modified gill-filament which acts as an operculum to the 

 tube is grooved on one side and serves as a brood pouch 

 (cf. p. 62). 



It is well known that several species of Nereis undergo 

 very considerable modification at the breeding season. The 

 changes affect mainly the parapodia, which become flattened 

 and oar-like, and their chaetse and to a certain extent other 

 organs as well. These changes are evidently related to the 

 pelagic habits which the worm (now called a Heteronereis) 

 adopts at this period, and ensure a wider distribution of the 

 sex cells than is possible to a bottom-living form. It is 

 only the posterior segments, containing the gonads, that 

 become modified ; the anterior segments remain unchanged. 



A similar change takes place in Syllids ; moreover, some 

 of the members of this family are further remarkable owing 

 to the fact that the hinder genital region of the body becomes 

 separated off, on maturity, from the anterior non-sexual 

 region (Benham, op. cit.). The original worm thus becomes 

 separated into a sexual and a non-sexual portion (schizogamy) 

 the former of which grows a head and the latter regenerates 

 a tail and again becomes sexual. In the simplest form of 

 this phenomenon, the growth of the new head and tail takes 

 place after the fission has occurred, but in some cases the 

 regeneration of the new portions may take place before the 

 separation, so that the original animal now consists of two 

 complete worms. Further, a narrow neck of proliferating 

 tissue may make its appearance in front and, becoming very 

 active, may bud off a number of new worms, all of which 

 become sexual. In this way, a chain of as many as sixteen 

 worms may be formed, e.g. Autolytus (Benham, loc. cit.). 

 The worms formed from a particular parent " stock " are 

 all of the same sex as that stock. Moreover, the difference 

 between male and female worms is so great as to cause 



