270 THE INVERTEBRATA 



which probably correspond to tentacles, are very numerous. Each 

 tentacle has a ciliated groove running from the tip to the mouth and 

 along this minute particles may be seen to travel. In the terebellids 

 these tentacles are extensible and capable of independent movement 

 when separated from the body. In the serpulids and sabellids, they 

 are rather stiff branched structures, which can, however, curl up 

 when withdrawn into the tube ; they sometimes bear eyes and some- 

 times are wonderfully pigmented. 



Besides the food-collecting tentacles there are gills in the tere- 

 bellids. These are branched processes, usually three pairs, situated 

 just behind the head, full of circulating blood. In the serpuHds and 

 sabellids, there are no special respiratory organs but the whole 

 surface of the body serves for the exchange of gases. 



In the terebeUids the tubes are composed of a soft cementing 

 substance mixed with mud or a parchment-like material to which 

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

 is usually porous (so that change of water can take place through it) 

 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 The tentacles are violently withdrawn in obedience 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. 188) 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 

 membrane f^ 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 



