344 ESSENTIALS OF ZOOLOGY 



The class Chaetopoda may be divided into two orders; namely, 

 (1) the Polychaeta and (2) the Oligochaeta. 



Order Polychaeta, — The polychaetes (many bristles) are typically 

 marine Chaetopoda. One of the most widely known forms of this 

 group is Nereis virens or the clamworm. It possesses many setae 

 (chaetae) located in fleshy parapodia. In this case the parapodia 

 with their setae constitute the segmented appendages. The parapodia 

 are used principally as locomotor and respiratory organs. 



The head of Nereis seems to have resulted from the fusion and 

 specialization of the anterior segments. It is composed of a prosto- 

 mium, which bears a pair of tentacles, a pair of palps, and two pairs 

 of eyes. The peristomium constitutes the first segment and bears four 

 pair of cirri or tentacles. The pharynx is equipped with muscles by 

 which it can be everted, and a pair of chitinous jaws which protrude 

 when the pharynx is extended. The jaws serve in capturing small 

 organisms and crushing anything which is to be swallowed. The suc- 

 ceeding segments are all alike except the posterior one which bears 

 a pair of ventral cirri extending posteriorly. 



The circulatory system is composed principally of a dorsal and a 

 ventral blood vessel joined in each segment by a pair of connecting 

 vessels. The blood is forced anteriorly through the dorsal vessel and 

 passes posteriorly through the ventral one. Its movement is effected 

 by wavelike contractions in the walls of the dorsal vessel. 



Each segment of the body except the peristomium has two nephridia 

 (excretory tubules) opening directly from the coelom to the outside. 

 The nephridia serve to convey the excretory products to the outside. 

 The sexes are separate and there are gonads in all the segments 

 except those in the anterior end of the body. The sex cells arise 

 from the walls of the coelom and when ripe pass to the outside, 

 fertilization taking place in the water. The fertilized egg develops 

 into a trochophore larva, which metamorphoses into the adult animal. 

 In the central nervous system there are two suprapharyngeal gan- 

 glia dorsal to the pharynx. These are connected by means of com- 

 ynissures to the suhpharyngeal ganglion ventral to the pharynx. A 

 nerve chain, composed of segmental ganglia joined by intersegmental 

 connections, extends posteriorly on the ventral side of the body to 

 the anal segment. Lateral nerves from the ventral nerve chain in- 

 nervate the various organs of the worm. 



Order Oligochaeta.— The best known example of the order Oligo- 

 chaeta is Lumhricus terrestris, the common earthworm, which is used 



