CHAPTER IX 



STRUCTURE AND CLASSIFICATION OF SEGMENTED 

 WORMS, SIPHON-WORMS, WHEEL-ANIMALCULES, 

 MOSS-POLYPES, AND LAMP-SHELLS 



SEGMENTED WORMS (Annelida) 



This phylum is a vast assemblage of marine, fresh-water, and 

 terrestrial worms and leeches, which agree with one another in 

 the possession of a bilaterally symmetrical body divided into 

 numerous similar segments, and in the absence of jointed appen- 

 dages like those of the Arthropoda. The body-wall consists 

 of skin with underlying layers of muscle; there is a muscular 

 pharynx; and the excretory organs are in the form of numerous 

 pairs of convoluted tubes (nephridia), each of which, in typical 

 cases, opens on the one hand to the exterior, and on the other 

 into a body-cavity of the same nature as that found in Verte- 

 brates (see p. 42). The phylum is divided into two classes: 

 I. Bristle- Worms (Chcetopoda), and 2. Leeches (Discophora). 



Class I.— BRISTLE-WORMS (Ch^topoda) 



A good typical example of this group is the so-called Sea- 

 Centipede (fig. 264), a name popularly applied to several species 

 of the genus Nereis which are found upon our coasts. The 

 body is bilaterally symmetrical and obviously divided into a large 

 number of segments, all very similar to one another except those 

 at the extreme ends. There are clearly none of the jointed lateral 

 appendages which are so characteristic of Arthropods, their place 

 being taken by hollow "foot-stumps" (parapodia), of which each 

 segment bears a pair. The nearest approach to such foot-stumps 

 among the Arthropoda are the legs of Peripatus (p. 399). Ex- 

 amination by means of a lens will show that each foot-stump is 

 divided into an upper lobe and a lower lobe near each of which 



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