244 



THE INVERTEBRATA 



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 most of the polychaets are usually patches of the 

 peritoneal epithelium, repeated in most of the segments, proliferating 



nephr. 



co'.d. 



nephr. nep'. 



Fig. 182. 



Fig. 181. 



Fig. 181. Segmental organ of Clepsine (Hirudinea). After Oka. Showing 

 mesodermal part with ciliated nephrostome and a single cell of the ectodermal 

 part, with intracellular duct. nu. nucleus. 



Fig. 182. Development of Megascolides australis (Oligochaeta). At the 

 posterior end the nephridia are single ; traced anteriorly they break up into a 

 number of loops each of which becomes a separate micronephridium {nep.'). 

 al. gut; sep. septa. Other letters as in Fig. 180 for both figures. 



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. 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. 



