ANNELIDA 



247 



These are connected around the oesophagus and give off three 

 longitudinal nerves, one dorsal and two ventral. There is a 

 pair of branched protonephridia which open on the side of the 

 body. The sexes are usually distinct. 



542. Phylum IV. Annelida. — The true worms are free 

 Hving, aquatic or, if terrestrial, at least confined to niuist 

 situations. The body is usually much elongated, bilaterally 

 symmetrical and segmented, and the segments are similar 

 (homonomous) . The intestine is usually a straight tube with 

 a vent at the posterior end of the body. There is a true body 

 cavity completely Kned with mesoderm. Eyes and other sense 

 organs are often present. The nervous system consists of a 

 supra-cesophageal ganghon, a pair of circum-oesophageal con- 

 nectives and a ventral chain of gangha arranged metamerically 

 and connected by a pair of longitudinal nerves. In each seg- 

 ment there is a pair of nephridia. 



543. Class II. Chaetopoda. — The Cha^topoda include the 

 typical worms, such as nereis and the earthworms. They are 

 distinguished by the cuticular bristles or seta^ with which each 

 segment of the body is armed. 



544. Order i. — The PolychcBta are marine annelids. They have two 

 bundles of setae on each side of each segment. The setae are borne by 

 short, unjointed appendages (parapodia) which are divided into two 

 branches, each branch having a bundle of setae. They usually live on the 

 sea bottom in burrows or tubes but some are pelagic. Many are act've 

 predatory animals and have well-developed sense organs. Others live in 

 leathery or calcareous tubes formed by secretions of epidermal glands. 

 These never leave the tubes voluntarily. Only the anterior end is in 

 most cases protruded for the purposes of feeding and respiration. In 

 many cases a circle of feather-like tentacles covered with cilia produce 

 currents by which food is carried to the mouth. The sexes are usually 

 distinct and there is a metamorphosis in development. 



545. Order 3. — The Oligochccta are the small fresh- water worms and the 

 earthworms. They lack parapodia and the bristles arc few in number, 

 usually eight in each segment, two groups of two each on a side. They 

 are hermaphrodytic and the development is direct. 



