RESPIRATION 147 



by modified sucker-like neuropodia and maintains a flow of water through 

 the tube by the co-ordinated beating of three large fans of the middle 

 region (Figs. 5.3 and 5.4). The water-current serves both for respiration 

 and feeding, and gaseous exchange takes place over the entire integument 

 (161). 



An echiuroid worm Urechis caupo creates a feeding and respiratory cur- 

 rent through its U-shaped burrow by means of peristaltic contractions of 

 the body wall (Fig. 14.6). Another worm inhabiting a U-shaped burrow 

 is the lugworm Arenicola marina found on sandy shores (Fig. 5.24). The 

 tail end of the gallery opens to the surface, while the head shaft is filled 

 with loosened sand which continually settles down as the animal feeds. 



Fig. 4.6. Sea Urchin Echinocardium cordatum in its Burrow 



Water is pumped through the burrow from the tail to the head shaft by 

 regular waves of contraction which pass anteriorly along the body (head- 

 ward irrigation, Fig. 4.7). The resultant flow of water provides a respira- 

 tory current and also serves to keep the sand in the head shaft loosened. 

 When the tide recedes the burrow may be left exposed and partly filled 

 with stagnant water deficient in oxygen. Under these conditions the lug- 

 worm sometimes shows another behaviour pattern, in which the hind-end 

 is coiled and thrust above the surface of the water and is then withdrawn 

 with a trapped bubble of air. This is brought into contact with the gills and 

 replenishes the oxygen supply of the animal. 



Other worms which create respiratory currents through tubes and 

 burrows are nereids, terebellids and sabellids. Some sabellid worms, e.g. 

 Sabella and Branchiomma, form firm tubes which are partially buried in 

 the substratum. The oral end of the tube extends above the ground, and 



