186 



COLLEGE ZOOLOGY 



Suckers .•• 



Adult in tube 



Tentacle 

 Mouth 



Seta 



Nofopodium 



Paropodia 



Food ball 

 feeding organ 



Dorsal view of 

 anterior end 



Figure 100. A marine polychaete (Chaetopterus) feeding in its tube. The arrows indicate the 

 direction of water currents. {Left after Enders; right after Lankester.) 



pumping fans, etc. In the feather duster 

 worms the peristomium or first segment is 

 enormously developed to form the feathery 

 tentacular crown, or food-gathering organ, 

 or to form also the operculum that serves 

 to close the end of the tube when the animal 

 is withdrawn. The tubes of the polychaetes 

 are nearly as variable and characteristic of 

 the different species as are the body parts; 

 the basic structures formed by the worms 

 may be spun threads (modified setal secre- 

 tions as in some of the scale worms), trans- 

 parent horny tubes, tough leathery tubes, 

 calcareous tubes, or clear glasslike tubes 

 (some serpulids). Extraneous materials such 

 as sand particles, shells, and sticks, are fre- 

 quently used and sometimes selected with 

 precision in regard to size, color, and weight, 

 so that some intelligence has been credited 

 to certain tube-dwelling worms. 



Polychaeta differ from Oligochaeta in be- 

 ing largely marine instead of fresh-water or 



terrestrial; parapodia are typically well de- 

 veloped, and the setae are numerous instead 

 of few; the prostomium or some of the first 

 few segments are often highly differentiated 

 to form a cephalic region of considerable 

 proportions; sexes are usually separate, with 

 gonads present in a large and variable num- 

 ber of segments. Fertilization of ova is 

 typically external; development is by spiral 

 cleavage and through a pelagic trochophore. 

 In certain species, for example Autolytus, 

 the body, which is only 15 mm. long, may 

 produce buds at the posterior ends, thus 

 forming a linear row of offspring (Fig. 101), 

 each of which acquires a head before sepa- 

 rating from the parent. There are thousands 

 of species of polychaetes. They are known in 

 all seas and at all recorded depths, but they 

 are most abundant in the upper 180 feet. 



The Pacific palolo worm, Eunice (Fig. 

 102), first became known from the Samoan 

 Islands, where it attracted the attention of 



