A^ 



CHAPTER 15 



pps"- 



Phylum Annelida. 

 Segmented Worms 



lnnelids (Fig. 90) are usually called seg- 

 mented worms in order to distinguish them 

 from flatworms and roundworms, which are 

 not segmented. The body consists of a 

 linear series of similar parts, which are 

 known as segments, somites, or metameres. 

 These are usually visible externally as rings; 

 the rings of an earthworm's body and the 

 vertebrae of man's backbone are evidences 

 of segmentation. 



Most annelids are marine, but many live 

 in fresh water, in the soil, or in other moist 

 places. Earthworms, sandworms, and leeches 

 are common examples. Most everyone is 

 familiar with the common earthworm for it 

 is distributed all over the earth except in 

 regions where the soil is nearly pure sand 

 and in mountain regions where the soil is 

 scanty and poor. For the most part earth- 

 worms are nocturnal. During the day they 

 are usually hidden in their burrows, but at 

 night they come out to feed. Apart from its 

 being not difficult to obtain, the earthworm 

 provides an opportunity for studying annelid 

 characteristics under advantageous condi- 

 tions. 



Metamerism, both external and internal, 

 is very conspicuous; the coelom is large and 

 obvious; several systems of organs, such as 

 the circulatory and nervous systems, are well 

 developed; and the details of behavior, re- 

 generation, and embryonic development are 

 well known. Several other annelids are very 

 briefly described; included in these are the 

 sandworm, Neanthes virens,"^ which is rep- 

 resentative of the class Polychaeta, and the 

 leech, Hirudo medicinalis, of the class Hiru- 

 dinea. 



170 



* There seems to be a rather common misconcep- 

 tion that Neanthes is the generic name for a group 

 of annehds formerly called Nereis. Both genera 

 Neanthes and Nereis are old, well established, with 

 many species attributed to each. Zoologists have 

 sometimes identified specimens as Nereis when more 

 critical determination would have shown that 

 Neanthes was correct. Figure 99 was drawn from 

 an actual specimen which came from a container 

 labeled Nereis, but it was positively identified by 

 Olga Hartman as Neanthes. 



