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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 

 living, aquatic or, if terrestrial, at least confined to moist 

 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 lined with mesoderm. Eyes and other sense 

 organs are often present. The nervous system consists of a 

 supra-cesophageal ganglion, a pair of circum-cesophageal con- 

 nectives and a ventral chain of ganglia arranged metamerically 

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

 ment there is a pair of nephridia. 



543. Class n. Chsetopoda. The Chaetopoda include the 

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

 distinguished by the cuticular bristles or setae with which each 

 segment of the body is armed. 



544. Order i. The Polychceta 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 active 

 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 Oligochata are the small fresh-water worms and the 

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

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

 are hermaphrodytic and the development is direct. 



