iS8 



ZOOLOGY 



taria they often correspond in number with the nephridia. 

 Their products ripen in the coelomic fluid (Fig. 98), and usually 

 escape through the nephridia, 

 they may however escape by 

 rupturing the body-wall. Im- 

 pregnation takes place exter- 

 nally. The eggs are sometimes 

 laid in small masses of jelly ; 

 sometimes they remain under 

 the care of the parent, under the 

 elytra in Polynoe, in a cavity in 

 the operculum in some Ser- 

 PULINAE, and attached to the 

 tube amongst the Terbbellidab. 

 Asexual reproduction is not 

 common ; it occurs, however, in 

 the Seepulidae and Syllidae. 

 In the former family a head 

 is formed by one of the seg- 

 ments in the middle of the 

 body, and the animal then 

 divides just in front of this. 

 In some of the Syllidae, 

 Autolyius, for example, one of 

 the posterior segments, usually the last, gives 

 rise to a new individual ; this may be repeated, 

 and chains of zooids are formed (Fig. 101). 

 These zooids break off, develope generative 

 organs, and reproduce sexually. As the original 

 worm was without sexual organs, this genus 

 exhibits an alternation of generations ; a very ^w. 102.— mreis 

 uncommon phenomenon in Chaetopods. It is After Oersted, 

 further complicated by sexual dimorphism, the 

 male worm beiug in many respects different in appearance from 

 the female. A somewhat similar phenomenon occurs in Nereis 

 (Fig. 102), one form being known as Seteronereis : this genus 

 is polymorphic, for in addition to the male and female forms, 

 hermaphrodite individuals also occur. 



Fig. 101.— Parent 

 stock of Autoly- 

 tus cornutus. 

 After A. Agassiz. 



