

Fig. 32-24. Parasitic trichina worm (Trichinella spi- 

 ralis), encysted in muscle. In man, a heavy infestation 

 of such roundworm parasites causes overt trichinosis, 

 a painful and long-enduring disease. (From General 

 Zoology, by Miller and Haub. Holt, Rinehart and Win- 

 ston, Inc.) 



of adult and larval parasites may impair the 

 lymph drainage, especially in the lower ex- 

 tremities, causing great swelling in the legs 

 (Fig. 32-25). The disease is restricted almost 

 completely to tropical regions and the inci- 

 dence is gradually declining as a result of 

 mosquito control. 



Phylogenetic Relationships. Among Ne- 

 mertinea, Nemathelminthes, and Rotifera par- 

 ticularly, the fossil record is scanty; and even 

 though the Brachiopoda and Bryozoa have 

 left many fossils, it is difficult to reach a 

 definite decision as to whether or not one or 

 more of these groups gave rise to others. 

 Most likely all represent early offshoots from 

 the same ancient extinct stock — the stock 

 that also gave rise to most of the higher 

 phyla (Fig. 29-11). 



THE ANNELIDA, OR SEGMENTED WORMS 



The earthworms (Fig. 21-3), sandworms 

 (Fig. 32-26), and leeches (Fig. 32-27) are per- 

 haps the most familiar members of the Annel- 

 ida. This phylum embraces almost 7000 

 known species, of which a large majority are 

 aquatic, mainly in marine waters. However, 

 there are some terrestrial forms and a few 

 parasitic species. 



Characteristics of the Annelida. Like many 

 other groups, the Annelida are triploblastic, 



The Animal Kingdom - 647 



bilaterally symmetrical animals with highly 

 developed tubular digestive tracts (Fig. 16-9) 

 and a clean-cut, fully lined coelomic cavity 

 (Fig. 16-10). The Annelida, however, display 

 a new feature, namely segmentation, which 

 does not appear in any of the earlier groups. 

 Segmentation, or Metamerism. The elongate 

 body of the typical annelid is conspicuously 

 subdivided into a series of segments, or 

 metameres (Fig. 32-26). Each of the succeed- 

 ing segments tends to be a close replica of 

 the others, not only as regards the external 

 features, but also as to the form and arrange- 

 ment of the internal structures. Such a seg- 

 mented organization is not restricted to the 

 annelids. It also is characteristic of the arthro- 

 pods and vertebrates. However, the segmenta- 



Fig. 32-25. Human leg, severely afflicted by ele- 

 phantiasis. This tropical disease, which is transmitted 

 by various mosquitoes, is caused by a parasitic nema- 

 tode, the filarial worm (Wucheren'a bancrofti). The 

 adult parasites live mainly in the lymph vessels and 

 lymph nodes of the body. (From Genera/ Zoology, by 

 Miller and Haub. Holt, Rinehart and Winston, Inc.) 



