BRANCH ANNULATA 



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." 1 



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

 divided into segments by muscular partitions or septa. 



These worms are bilaterally symmetric. The body is usually 

 elongated. 



CLASS I. CKETOPODA 



Class ChaetSp'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 divided 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, but 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'bricus) has an elongated cylindric body 

 of many segments or metameres. 



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



rounded, lobe-like projection, the prostomium. The moutl^p 



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



thick-walled, muscular pharynx. This pharynx can 



1 Galloway's " Zoology." 



