PHYLUM ANNELIDA 233 



Polygordius is a marine worm living in the sand. It is about 

 an inch and one half long, and only indistinctly segmented 

 externally. The prostomium (Fig. 162, h) bears a pair of ten- 

 tacles (ct.). The mouth opening is in the ventral part of the first 

 segment, and the anal opening (an\ in the last segment. A pair 

 of ciliated pits, one on either side of the prostomium, ^probably 

 serve as sense-organs. 



Internally Polygordius resembles the earthworm, but in some 

 respects is more primitive. The ccelom is 

 divided into compartments by septa. The 

 internal organs are repeated so that almost 

 every segment possesses ccelomic cavities, 

 longitudinal muscles, a pair of nephridia, a 

 pair of gonads, a section of the alimentary 

 canal, and part of the ventral nerve-cord. 

 The development of Polygordius includes a 

 trochophore stage. As shown in Figure 162, B, 

 the trochophore larva at first resembles a top 

 with cilia around the edge, an eye-spot (e), and 

 a digestive tract with both mouth (m) and 

 anal (an) openings. This larva resembles the 

 Pilidium larva of the NEMERTINEA (Fig. 118) 

 and certain adult rotifers (Figs. 122-123). 

 The adult develops from the larva by the 

 growth and elongation of the anal end as 

 shown in Figure 162, B, C. This elongation 

 becomes segmented (D) and by continued 

 growth transforms into the adult (A). 



4. CLASS II. CH^ETOPODA 



The CH^ETOPODA are annelids which possess 

 conspicuous setae. Two subclasses are recog- 

 nized: (i) the POLYCH^TA, like Nereis (Fig. 

 163), with many setae situated on paired fleshy 

 outgrowths, the parapodia (Fig. 164, para], and 



