278 THE INVERTEBRATA 



The closed nephridium appears to be the most primitive type of 

 segmental organ and a survival of the time when the coelom had not yet 

 developed. The open nephridium is far commoner in the Chaetopoda 

 with their extensive coelomic cavities. The origin of the coelomoducts 

 is doubtful. They may be thought to have arisen as genital ducts but 

 now the nephridia often serve for the escape of the gametes. 



The gonads in the polychaets are usually patches of the peritoneal 

 epithelium, repeated in most of the segments, proliferating until 

 a great number of the germ cells have been detached into the 

 body cavity which they almost entirely fill and where they undergo 

 maturation (Fig. 199 A). When ripe they reach the exterior usually 

 through the segmental organs, but occasionally the body wall ruptures 

 and so opens a way of escape. 



Like so many other marine animals the polychaets thus liberate 

 eggs and sperm freely into the sea, fertilization taking place externally. 

 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 irregu- 

 larly 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. 194 E) has 

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

 the October-November moon dawns the Pacific Palolo breaks oflf 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 arti- 

 ficially. 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 Autolytus (Fig. 

 194 B) and Myrianida 2i 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 



