POLYCHAETA. 471 



for the formation of the new segments begins before the two parts of 

 the worm separate, so that the posterior sexual worm acquires its 

 head while still attached to its sexless nurse. In other words a zone 

 of fission is formed, and this zone may produce not only the head of 

 the hinder worm, but a number of segments which become marked 

 out into a series of zooids in which genital organs eventually appear. 

 In this way a chain of sexual zooids, attached to the asexual "scolex" 

 in front, is formed ; the zooids are all of the same sex, and the 

 posterior of them is the oldest and most developed (Fig. 375, IV, V). 

 The male zooids differ very remarkably from the female, and were 

 formerly relegated to a distinct genus called Polybostrichus (Fig. 374), 

 the female being known as Sacconereis and characterized by the 

 possession of a ventral brood-sac. In this case we have a combina- 

 tion of fission, by which the first-formed zooid is formed, and 

 gemmation, by which, the succeeding worms are formed from the 

 segments successively budded off at the zone of fission. 



In Myrianida a very similar process takes place, and a chain of 

 sexual zooids attached to an anterior sexless zooid is formed ; in this 

 case, however, there is no fission, but gemmation only from a zone of 

 fission, for none of the segments of the original worm, except the 

 anal, enter into the constitution of the first formed sexual zooid ; 

 the process starts with the formation, in the gemmiparous zone 

 between the penultimate and anal segment, of new segments, which 

 give rise successively to the first formed and all subsequent zooids 

 of the chain. In this case, as in Autolytus, the males and females 

 are never formed on the same stock, and are distinguished as 

 Poli/bost rich us and Sacconereis forms (Fig. 376). Moreover, in each 

 zooid there is a region in which new segments are formed, so that 

 the posterior and oldest zooid of the chain is also the largest. 



Syttis ramosa a form obtained by the " Challenger " in the canal- 

 system of an Hexactinellid sponge possesses the peculiar property 

 of forming lateral buds, which may themselves produce buds before 

 separation : in this manner a branched colony arises, some of the 

 individuals of which develop genital organs, and probably become 

 free. 



Almost all Polychaets are marine, but a few fresh-water forms 

 are known, e.g., a Nereis and Lumbriconereis from Trinidad (Kennel), 

 Manayurikia in N. America (Leidy). A few are parasitic, e.g., 

 Oligognathus bonelliae (one of the Eunicidae] in the body-cavity 

 of Bonellia ; Labrorostratus parasiticus, another Eunicid, in the 

 body-cavity of Odontosyllis denostomatus ; larvae of the Alciopinae 



