CHAPTER X. 



ANNELIDA. 



I.— CH/ETOPODA AND ARCHIANNELIDA. 



The two chief divisions of the Chsetopoda are unlike as re- 

 gards development, this being in the Polychseta generally 

 indirect, and involving a free-swimming larval stage, where- 

 as in the Oligochseta it is considerably abbreviated, and 

 free-swimming larv« are absent. The Archiannelidi are like 

 the Polychaeta. 



1. Development through Free-swimming Larvae 



(Foltjchdeta and ArcTiiannelida). 



In general the Polychaeta develop from free-swimming 

 larvse which are provided with ciliated bands. Only a few 

 forms bring forth living young. Such is the case in Eunice 

 sanguinea, SylUs vivipara, and in a Cirratulus in which the 

 eggs develop either in the body cavity or in the cavity of a 

 segmental organ which has become a uterus. A kind of 

 brooding also occurs in many forms, as, for example, in Auto- 

 lytus cornutus, an extensive sac, in which the eggs are 

 developed, being formed on the ventral surface by the dis- 

 tension of the skin of the body. In Polynoe cirrata, the eggs, 

 which are stuck together in a single mass, are carried on the 

 dorsal surface under the dorsal scales. Similar to this is 

 Grubea limhata, in the females of which at the time of sexual 

 maturity the entire back is thickly covered with eggs, while 

 Exogone gemmifera and Sphasrosyllis pyrifera carry their eggs 

 on the ventral side, namely on the ventral cirri (Viguier, 

 No. 46). In Spirorhis Pagenstecheri the tentacle which bears 

 the operculum of the tube is enlarged, and thus serves as a 

 brood-chamber ; in Spirorbis spirillwin, on the contrary, the 



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