BRANCH Mm^JLATk 



The branch Annula'ta is distinguished from the other branches 

 of worms by having external and internal segmentation, that is, 

 being divided into rings or segments (metameres) " containing 

 homologous organs or similar portions of a continuous organ."^ 



They have, usually, a well-developed coelom or body cavity, 

 divided into segments by muscular partitions or septa. 



These worms are bilaterally symmetric. The botly is usually 

 elongated, 



CLASS I. CH^TOPODA 



Class Chsetop'oda consists of fresh-water and marine annelids 

 which bear setae, or bristles. The setae arise from special fol- 

 licles, and may occur singly or in bunches. These setae, which 

 are controlled by special muscles, act as tiny levers in locomo- 

 tion. 



They have a body cavity which is partially divick'd into com- 

 partments corresponding to the segments. The alimentary 

 tube extends through the body and is usually constricted at the 

 septa. There is usually a well-developed circulatory system. 

 Respiration is usually through gills or branchiae and through 

 the body wall. In some forms the sexes are distinct, while 

 other forms are hermaphroditic. Fresh-water annelids de- 

 velop without a metamorphosis, l)ut in many marine forms the 

 trochosphere larvae occur. 



Few are true parasites, but a number are commensal, habitu- 

 ally associating with other animals for their food and shelter. 

 Many sea-worms are phosphorescent. 



The earthworm {Lum'hricus) has an elongated eylindric ])ody 

 of many segments or metameres. 



Digestive System (Fig. 50). — The mouth is covered by a 



rounded, lolx'-like ])rojection, the prostomium. The mouth 



leads into a small buccal cavity, back of which is the larger. 



thick-walled, muscular pharynx. This pharynx can be pro- 



^ Galloway's " Zoology." 



5 65 



