SURVEY OF INVERTEBRATES 81 



to insure entrance to the proper host. Before a parasite can enjoy 

 Utopia, it must get there. (Figs. 44, 254, 255.) 



E. Segmented Worms 



The Segmented Worms form a relatively straightforward phy- 

 lum, the Annelida, that carries us on apace in the development 

 of the more complex animal body: conspicuously by definitely 

 introducing the principle of segmentation. Well-known repre- 

 sentatives of the chief groups are the marine Sandworms and 

 Tubeworms, or Polychaeta; the Earthworms, or Oligochaeta, 

 of soils the world over; and the Leeches, or Hirudinea, such as 

 the Medicinal Leech, so popular when blood-letting was in vogue. 

 (Figs. 45, 60.) 



The body of the Sandworm, Nereis, consists of a linear series 

 of some fifty units, or segments. Several of the anterior ones 

 form the head, with sense organs, notably rather complex eyes, 

 mouth, and chitinous jaws; while most of the rest are provided 

 with paddle-like, bristled parapodia that act both as respiratory 

 and swimming organs. The anus opens on the terminal segment. 



The Earthworms show a simplification of the external structures 

 present in Sandworms, because the head is far smaller and simpler 

 and lacks specialized sense organs. Moreover the bristles (setae) 

 are all that remain of the conspicuous locomotor organs of the 

 Sandworm. The largely sedentary and nocturnal life of Earth- 

 worms renders highly specialized locomotor, respiratory, and 

 sense organs unnecessary. But the internal organs of both follow 

 the same general plan, and since we shall have occasion later 

 to study those of the Earthworm somewhat in detail, it will 

 suffice now merely to emphasize the essential progressive features. 

 (Fig. 45.) 



The principle of segmentation, as already mentioned, is probably 

 the most significant advance made by the phylum. At all events, 

 it is the plan of organization exemplified by the most successful of 

 the higher groups — the Arthropods and Vertebrates. Apparently 

 the segment is an adaptable unit of organization that makes possible 

 local specialization in higher forms, and so contributes in an im- 

 portant way to their response to various environmental changes. 

 Furthermore, in Segmented Worms the coelom first attains high 

 structural significance and affords ample space for the disposal of 

 more elaborate organ systems. Thus we find a complete circula- 



