416 



GENERAL ZOOLOGY 



In some annelids the head of the adult later increases in size by- 

 taking up trunk segments. In monoecious annelids such as the 

 earthworm, the egg develops directly into a worm, but the pro- 

 stomium shows such a close resemblance to the trochophore larva 

 that the two structures are thought to be homologous. The 

 trochophore larva is important in connection with the evolution 

 of the annelids, since it bears a rather striking resemblance to a 

 rotifer and an even closer resemblance to the larva of the Brachio- 



A b c 



Fig. 241. — Polygordius. A and B, two stages in the development of the 

 trochophore (trochosphere) larva which grows by the addition of segments to the 

 posterior end. C, dorsal view of the adult, a, anus; g, rudimentary supra- 

 esophageal ganglion; m, mouth; n, protonephridium, provided with flame cells; 

 T, tentacles. {After Hatschek and Fraipont.) 



poda and the Phoronidea and also to the veliger larva of the 



Mollusca (p. 433). 



CLASS I. ARCHIANNELIDA. Primitive marine annelids. 

 Setae (locomotor bristles) absent or scanty. Head may bear 

 ciliated bands or a pair of tentacles. External segmentation 

 often indistinct. A trochophore larval stage occurs. All 

 dioecious. 



Example: Chaetogordius canaliculatus, has setae on its 

 posterior segments. Common at Woods Hole. 



CLASS II. POLYCHAETA. Mostly marine annelids, with a 

 few fresh-water species. They are free-swimming or tube 



