188 



COLLEGE ZOOLOGY 



A tube-dwelling species is Chaetopterus 

 (Fig. 100). When full grown it may reach 

 15 to 30 cm. (12 inches); the body is highly 

 luminescent and consists of three distinct 

 regions. The U-shaped, opaque, parchment- 

 like tube may be 50 cm. long; it lies com- 

 pletely hurried in mud or sand, except for 

 the two distal orifices. The worm maintains 

 its position in the spacious tube by means 

 of the long anterior notopodia (Fig. 100) 

 and three ventral suckers that are formed by 

 the median fusion of three pairs of neuro- 

 podia. A powerful current of water may be 

 set up by three muscular fan segments, 

 formed by the median fusion of three pairs 

 of notopodia. Other remarkable modifica- 

 tions include the food-ball organ (Fig. 100) 

 that is formed by the fusion of a pair of 

 notopodia and serves to carry mucous food 

 balls to the mouth. This polychaete is world- 

 wide in its distribution; it usually occurs 

 where there are broad sand flats and little 

 current. It is found along the Atlantic Coast 

 from North Carolina to Cape Cod. 



Archiannelida 



The Archiannelida are aberrant marine 

 polychaetes, characterized largely for the per- 

 sistence of larval features such as ciliar)' 

 rings and lack of setae, or reduction of organ 

 systems. Whether these traits are primitive 

 or degenerate is not known. Polygordius ap- 

 pendiculatus (Fig. 103) lives in the sandy 

 shores of the Atlantic and Mediterranean 

 coasts. It is about one inch long and indis- 

 tinctly segmented externally. The prosto- 

 mium bears a pair of cephalic tentacles, and 

 the posterior end bears two anal tentacles. 

 A pair of ciliated pits, one on either side of 

 the prostomium, probably serve as sense or- 

 gans. The development of Polygordius in- 

 cludes a trochophore stage. The adult 

 develops from the trochophore by the 

 growth and elongation of the anal end. This 

 elongation becomes segmented; and, by con- 

 tinued growth the larva transforms into the 

 adult. 



The archiannelids number only about 45 



Cephalic 

 tentacles 



Head 



Faint exfernol segmentation 



Young trochophore 

 larva 



Elongation of 

 trochophore larva 



Transformation stages of larva 



Figure 103. Stages in development of Polygordius appendiculatus. one of the Archiannelida. 

 (After Fraipont.) 



