274 POLYCHAETA CHAP. 



which must be added some Hesionidae. In this family ova 

 and spermatozoa are developed around the same blood-vessel. 

 But in the former group of worms (as also in Ophryotrocha) 

 the two kinds of cells are produced in different regions of the 

 body. Thus in Protula the anterior abdominal segments are 

 male, the posterior ones female, while in Spirorbis the reverse 

 arrangement holds ; and in Syllis corruscans the anterior 

 segments of the body contain eggs, whilst the posterior region 

 contains spermatozoa, and this region separates and becomes a 

 male worm. 



The eggs and spermatozoa in the Polychaeta are discharged 

 into the sea either by rupture of the body-wall or through the 

 nephridia ; the male and female elements unite, and the result- 

 ing fertilised eggs undergo development, either floating separately 

 in the water, or embedded in jelly, or attached to the body or to 

 the tube of the worm. 



The result of the segmentation of the egg is a free-swimming 

 larva known as a " Trochosphere," similar to that of Polygordius. 

 The larvae of different species present various more or less 

 marked departures from this type, for instead of the two girdles 

 of cilia there may be only the anterior girdle, or there may be 

 several complete or incomplete girdles between the two typical 

 ones, or there may be (Chaetopterids) only a single girdle of cilia 

 about the middle of the body, the two typical girdles being absent. 1 

 The postoral region, after elongation, generally becomes marked 

 out into three segments, and these segments develop chaetae, 

 which are usually temporary and specially long. 



The little animal is thus equipped for an independent life : 

 the provisional chaetae help in keeping it balanced ; and in some 

 cases (Spionidae) serve to protect the little soft creature, for when 

 it is touched it curls up, and its chaetae stick out at the sides, so 

 that it looks like a hairy caterpillar. But the larva is quite at 

 the mercy of the sea, for it is carried hither and thither by 

 currents, and in this way the species is disseminated. The 

 larvae of the Polychaetes, like those of other animals, occur at 

 certain periods of the year in large quantities at the surface of 

 the sea, and serve as food for various larger animals. 



1 See Claparede and Metschnikoff, "Beit, ztir Kennt. d. Ent'.vick der Chaeto- 

 poden," Zcitschr.f. wiss. Zool. xix. 1869, p. 163 ; and Fewkes, "On the Develop- 

 ment of certain Worm Larvae," Bulletin Mus. Harvard, xi. 188b, p. 167. 



