ANNELIDS IN GENERAL 



253 



290. Reproduction. — Reproduction has been referred to in the case 

 of the earthworm and of the leech. Sexual reproduction in other forms 

 is similar. There is, however, a type of asexual reproduction by means 

 of transverse fission which sometimes occurs in annelids, the result of 

 which is to produce a colony of several individuals moving about together 

 (Fig. 159). In Aeolosoma this is the ordinary mode of multiplication, 

 and sexual reproduction seems to be rare. Budding is also exhibited by 

 certain Chaetopoda. In some of them lateral buds form on certain 

 segments, while in others the budding is restricted to the undersides of 

 the last two segments. In most 

 cases the buds eventually break 

 away from the old worm and become 

 new individuals. 



The sex cells of most polychaets 

 are reproduced from the epithelium 

 lining the coelom and either pass 

 into the water through the nephridia 

 or escape through ruptures in the 

 body wall. In connection with this 

 liberation of the sex cells, the palolo 

 worm (Fig. 160) of the Southern 

 Pacific Ocean exhibits a phenome- 

 non known as swarming. This oc- 

 curs regularly every year beginning 

 on the first day of the last quarter of 

 the October-November moon and 

 continues for two or three days. 

 The worms which lie in burrows at 

 the sea bottom have their whole ^2/ ^^^^ coY^esy o/ i/mr^, //o/< * Companj/.) 



Ine budded individuals develop in order, 

 posterior portions modified for the the one at the end of the chain being the 



production and retention of the sex °|"^e!*: compare with formation of pro- 



glottids m the tapeworm. 



cells. When the day for swarming 



arrives the reproductive portions break away and rise to the surface in 

 such numbers that the surface of the sea is covered with the writhing 

 mass. In a short while the sea becomes milky with the millions of 

 liberated egg and sperm cells. The anterior portions of the worms 

 remain at the bottom to regenerate another reproductive portion for the 

 following year. A similar palolo worm in the West Indies region swarms 

 in the third quarter of the June- July moon. 



In one family of the polychaets the sexual zooid does not at once 

 separate; both it and the asexual one multiply by transverse fission and 

 this continues until a chain of as many as 16 individuals is produced, the 

 anterior ends of each of which are asexual and the posterior sexual. All 

 of the individuals in a chain are of the same sex. 



Fig. 159. — A chain of individuals 



formed in an annelid, Myrianida sp., by 



budding. {From Hertwig and Kingsley, 



'Manual of Zoology," after Milne-Edxvards, 



