232 THE BIOLOGY OF MARINE ANIMALS 



material for nourishment. Such animals possess gullets, tentacles and 

 similar structures which can be pushed through the ground, whereas 

 organs of mastication are absent. 



Annelids. Many polychaetes fall into this group, such as Arenicola, 

 Ophelia, Notomastus and others. Arenicola prefers muddy sand in which it 

 forms a U-shaped burrow. This consists of head-shaft, gallery and tail- 

 shaft (Fig. 5.24). Usually only the latter is open to the surface, while the 

 head-shaft is blocked by loosened sand. When the worm finds a suitable 

 location it may stay there for weeks on end, showing a very regular pattern 

 of activity, at least when the tide is in. Sand is loosened in the head-shaft 

 by regular irrigation movements, and is swallowed by the muscular pro- 

 boscis which is frequently extended and withdrawn. Feeding activity is 



Fig. 5.24. Lugworm Arenicola marina in its Burrow 



The cross-lines lie at the boundaries between head shaft {left), gallery (below) and 

 tail shaft (right). Yellow sand above, black sand below. Solid arrows show water move- 

 ments through burrow; broken arrows, settling of sand in the head shaft. (After Wells, 

 1945.) 



rhythmic, showing a periodicity of about 7 min in A. marina (Fig. 4.7, 

 p. 148) (101, 102). 



Sipunculoids also ingest sand and mud by means of a muscular in- 

 trovert and utilize the contained organic material. Ingestion of bottom 

 deposits is more refined in Cirratulus. This animal burrows in mud and 

 sand but is able to select diatoms and other algal and organic material 

 from the non-nutritive mass. Tentacles are present but are exclusively 

 respiratory. Sensory flaps on the anterior pharyngeal walls permit the 

 passage of only the finest particles, which apparently are sucked into the 

 gut. 



Echinoderms include many benthic and burrowing species which feed 

 on bottom deposits. Holothurians push mucus-bound aggregations of 

 bottom materials into the mouth with the buccal tentacles. Heart urchins 

 (spatangoids) burrow in sand or mud and maintain communication with 

 the surface by a mucus-lined canal (Fig. 4.6, p. 147). Through the latter 

 a respiratory current is drawn, while small rosette feet of the buccal region 



