THE PHYLUM ANNELIDA 



(1) the simple, generalized condition of metamerism and (2) the development 

 and relationships of the coelom and its derivatives. In comparison with the 

 acoelomate, non-metameric turbellarians, for example, the annelid plan is 

 considerably advanced. Comparison with a more complex metazoan, such as 

 an arthropod or a vertebrate, shows that the relative complexity of these 

 higher forms is largely a difference in degree. The fundamental characteris- 

 tics of metamerism and of the coelom are firmly established in the annelid; 

 the more advanced structural features of arthropods and of vertebrates may 

 be interpreted as elaborations and modifications of a basic plan already laid 

 down in the annelids or in their ancestral stock. This is the more interesting 

 when we reflect that both segmentation and the coelom apparently originated 

 differently and evolved independently, in the annelid-arthropod stock and in 

 the echinoderm-chordate line (see Fig. 7.3, p. 219). In any case, in none of 

 the metazoan phyla more advanced than the annelids are the characteristics 

 of metamerism and coelom present in such an uncomplicated condition as in 

 these segmented worms. 



Even within the phylum Annelida, evolution has involved departures from 

 the primitive conditions in these characteristics. The process of cephalization, 

 the development of anteroposterior differentiation, has progressed to a con- 

 siderable degree in the higher annelids. Along with cephalization there 

 has been an increase in regional specialization. As a result, the originally 

 uniform series of similar somites has become organized into a series of 

 differentiated groups of somites, each group modified for the performance 

 of some particular function. This has involved general structure, the append- 

 ages, and the internal organs as well, and it is perhaps best demonstrated 

 among the sedentary, tube-dwelling polychaetes. The oligochaetes and 

 leeches furnish examples of variant modifications of the originally undifferen- 

 tiated and extensive coelomic pouches. In specifically delimited regions of the 

 bodv these have been given over to reproductive functions; in the Hirudinea 

 they have all but disappeared. 



Other annelid characteristics should be emphasized in anticipation of our 

 discussions, in the next chapter, of the phylum Arthropoda. Throughout 

 their history, the annelids have retained and developed, as the basic locomotor 

 mechanism, the body-wall musculature, consisting of concentric sheets of 

 antagonistic circular and longitudinal muscle fibers. In correlation with the 

 type of locomotion to which they are thus limited, and with their cutaneous 

 method of respiration, the annelids have retained the thin, flexible, secreted 

 cuticle as a protective cover for the body wall. The nervous system, organized 

 on the basis of quasi-independent segmental units, is also significant. All 

 the.se characteristics, as we shall see, foreshadow special conditions to be 

 encountered among arthropods. 



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