GENERAL ZOOLOGY 



tracks" from the mud flats of ancient seas. The earhest fossil annehds 

 date from the Cambrian period (roughly 500 million years old) and indicate 

 that considerable differentiation had already taken place at that time. 



There are few annelids in which the size of the individual exceeds that of 

 the average earthworm. Their economic significance is not great, except as 

 they form an integral part of the food supplies of larger aquatic animals, as 

 well as of birds and mammals. Of more direct importance to man is the 

 activity of earthworms in improving soil texture. Populations of these worms 

 may exceed 50,000 per acre, and they tunnel through and turn over the soil 

 very effectively. 



For comparative studies of animals, the annelids are important because of 

 their simple metameric structure. The present chapter describes the clam- 

 worm and the earthworm as representative annelids, with particular atten- 

 tion given to the earthworm. This animal exhibits a structural and functional 

 organization which may be considered intermediate in complexity between 

 that of the hydra and that of the vertebrate. 



The Class Polychaeta 



The ;iame Polychaeta ("many bristles") is appropriate for members of this 

 class, the typical polychaete having on each segment a pair of appendages 

 stiflTened and fringed by many bristle-like structures. The polychaetes are 

 almost exclusively marine forms and are abundant in all the oceans. Accord- 

 ing to the species, they are found crawling or burrowing upon the bottom, or 

 inhabiting secreted tubes, or even swimming freely at the surface. The 

 earliest annelid fossil remains are of worms recognizable as polychaetes. 

 Taken as a whole, the polychaetes best typify the annelid level of organiza- 

 tion; they include generalized forms as well as many that are highly 

 specialized. 



The Clamworm: Nereis virens. As well as any species that can be 

 selected, the clamworm represents the polychaetes and the entire phylum 

 (Fig. 14.1). This animal lives just below low-water mark in sand and fine 

 gravel, or under stones, without forming permanent burrows. When free in 

 the water, it swims effectively by lateral undulations of the body, assisted by 

 the paddle-like appendages; at other times it creeps on the substratum, "walk- 

 ing" on its segmental appendages. The animal customarily burrows into the 

 bottom, lying with only the head exposed. The mouth opens ventrally at the 

 anterior end and is encircled by the first somite, the peristomium. The dorsal 

 surface of the head region is well equipped with numerous sense organs in the 

 form of eyes, tentacles, and palps (Fig. 14.2). The anus, flanked by two 

 anal cirri, which are tactile organs like the tentacles of the head, lies at the 

 extreme posterior end. The body is composed of somites, each bearing a pair 

 of paddle-like parapodia, which are the organs of locomotion and gaseous 

 exchange. Each parapodium consists of dorsal and ventral leaf-like lobes, the 



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