246 THE INVERTEBRATA 



This habit is associated in many forms with the phenomenon of 

 swarming in which a worm, usually crawling or burrowing on the 

 sea bottom, when sexually mature rises to the surface and swims 

 vigorously, eventually discharging its genital products and sinking 

 to the bottom as suddenly as it rose. In most nereids this occurs ir- 

 regularly through the summer months , but in at least two forms {Leodice 

 viridis^ the "Palolo" of the reefs of the Southern Pacific, and Leodice 

 fucata of the West Indian reefs) the phenomenon (Fig. 183 E) has 

 acquired the strictest periodicity. As the day of the last quarter of 

 the October-November moon dawns the Pacific Palolo breaks off the 

 posterior half of its body, already protruding from the mouth of its 

 burrow in the coral rock, and these fragments rise to the surface in 

 such quantities that the water writhes with worms and is later milky 

 with the eggs and sperm discharged. Immediately afterwards the 

 remaining anterior end begins to regenerate the missing portion, 

 but a whole year elapses before the gametes are again ripe — even 

 two days before spawning occurs fertilization cannot be brought 

 about artificially. In the West Indian species the phenomenon 

 is similar but takes place in the third quarter of the June-July 

 moon. 



In the syllids the phenomena of swarming are vastly more varied. 

 The whole animal may produce germ cells and swarm. Usually how- 

 ever the gonads are confined to the posterior part of the body which is 

 detached as a free-swimming unit ; this often develops a head but never 

 jaws and pharynx. It can live for some time but not feed. In the 

 majority of forms a single bud is produced, but in ^m/o/j^m^ (Fig. 183 B) 

 and Myrianida a proliferating region is established at the end of the 

 original body and from this a chain of sexual individuals is budded 

 off, the oldest being situated most posteriorly. The whole chain may 

 be found swimming at the surface, the original worm dragging after 

 it the chain of sexual individuals which one by one detach and lead 

 a short independent existence. In some species of Trypanosyllis 

 (Fig. 183 C) the zone of proliferation is in the form of a cushion of 

 tissue on the ventral surface of the last two segments and this produces 

 not a linear series of buds but successive transverse rows, amounting 

 to more than a hundred — the fully formed sexual individual possesses 

 a head but no vestige of an alimentary canal. The extraordinary 

 branching form, Syllis ramosa (Fig. 183 D), shows remarkable capacity 

 for heteromorphic growth in the production of sterile side branches 

 from the stock and reproductive buds. 



In the syllid there is usually no notopodium during asexual life 

 but during the maturation of the gonads the parapodium is recon- 

 structed, a notopodium being formed from which spring bundles of 

 long capillary swimming chaetae, while a corresponding develop- 



