ANNELIDA OR ANNULATA loi 



5. The nervous system consists of two dorsal ganglia, connecting 



commissures passing around the pharynx, and a ventral 

 chain of ganglia with lateral nerves. 



6. The alimentary canal is well developed and usually specialized. 



7. Trochophore larva found in many forms. 



Natural History 



Class I. Archi-Annelida. — The most primitive of the Annelida 

 are the Archi-Annelida, represented by two families, both marine 

 and exceedingly small. The family Folygordiidae includes the 

 sand-living form Polygordiiis. It is slightly over an inch long, with 

 indistinct external annulations, but a septate coelom, and metameric 

 development of nephridia, gonads, digestive tube and ventral nerve 

 cord. The larva has a trochophore stage. 



The family Hist7~iodrilidae are minute parasitic worms infesting 

 the lobster. They have three horny jaws, a well-developed digestive 

 system consisting of esophagus, intestine and rectum; and have 

 primitive united cerebral ganglia. The sexes are separate. 



Class 2. Chaetopoda. — In the Chaetopoda, segmentation is 

 distinct both internally and externally, and the setae are segmentally 

 arranged on the parapodia or sunk in pits. 



Order 1. Polychaeta. — The polychaetes are chiefly marine 

 animals, with setae arranged in groups on the fleshy parapodia. 

 They have a distinct head, usually provided with sense organs. 

 The prostomiufn bears from one to ten dorsal tentacles and two ventral 

 palps, which in certain forms are broken up into long respiratory 

 filaments. The sexes are usually separate. Eyes are present on 

 the prostomium of some forms, and lithocysts in a few forms (^reni- 

 cola). Polychaetes are of diflferent colors, including red, blue, green, 

 or yellow. They usually pass through a trochophore stage. (See p. 

 125.) 



Heterogony is present in Nereis, a small pelagic form alternating 

 with a large bottom living one. The palolo worms of Samoa 

 {Leodice viridis) come to the surface during the October full moon to 

 breed and are caught by the natives who use them for food. 



A few polychaetes are found in fresh water; the rest are marine, 

 and chiefly bottom living animals, which burrow in the sand or live 

 in tubes. The free-living forms are predaceous; the sedentary ones 

 Hve on all kinds of organic matter. There are many commensals 

 and a few parasites. (See p. 483.) 



