POLYCHAETA 237 



adhere sand grains, sponge spicules, foraminifera or fish-bones. It 

 is usually porous and the animal occasionally leaves its shelter ; there 

 are at least two openings to the exterior. The tube of the chaetopterids 

 is parchment-like but in the serpulids there is a groundwork of 

 mucin in which carbonate of lime is laid down. In the latter family 

 there is only one opening from which the crown of tentacles emerges 

 but never any more of the body. The tentacles are violently with- 

 drawn in obedience to any such stimulus as touch or change of 

 illumination. 



In all the types except Chaetopterus the body is divided into two 

 regions, an anterior thorax and a posterior abdomen. The thorax is 

 composed of segments in which the notopodium is a conical structure 

 with capillary chaetae while the neuropodium is a vertical ridge in 

 which are imbedded short-toothed chaetae called uncini, which only 

 just project from the body wall. It is suggested that the notopodium 

 assists movement up and down the tube while the neuropodia are 

 braced against the tube and maintain the worm in position. In the 

 abdomen the arrangement of the parapodia is different, and in the 

 serpulids and sabellids the uncini become dorsal and the simple 

 chaetae ventral (introversion). 



In the serpulids (Fig. 177) the peristomium is similar to the 

 other thoracic segments but it is produced into a collar which folds 

 back over the ventral surface and sides and secretes successive hoop- 

 shaped rings which are added to the tube. Other features are the 

 thoracic mew^rawe, a lateral frill possibly respiratory, and the operculum^ 

 a much enlarged and stopper-like branch of a tentacle which exactly 

 closes the mouth of the tube when the animal is retracted. 



The renewal of water round the body is of the utmost importance 

 in respiration. It is brought about by undulatory movements of the 

 abdomen and sometimes by sharp rhythmic contractions and ex- 

 pansions of the body which pump the body in and out of the tube. 

 The great development of the dorsal bands of longitudinal muscle seen 

 in a transverse section of a serpulid (Fig. 175 C) is characteristic of the 

 tubicolous worm. Another typical modification seen in the serpulids 

 and sabellids is the median ciliated groove, which starts from the anus, 

 runs along the ventral surface of the abdomen, turning on to the 

 dorsal surface when the thorax is reached. It serves to conduct the 

 faeces to the mouth of the tube. 



Chaetopterus (Fig. 176 B) is probably the most modified of all 

 tubicolous worms. It lives in a parchment-like tube which is U- 

 shaped with at least two apertures. There is a peristomial collar as in 

 other tubicolous worms, but the tentacles are a pair of rudimentary 

 processes. A very complicated mechanism exists for obtaining food, 

 which can be observed by taking a live Chaetopterus from its tube and 



