148 THE BIOLOGY OF MARINE ANIMALS 



the caudal end sometimes reaches the surface, sometimes is buried. In 

 either event it is perforate at the posterior end. Water currents are driven 

 through the tubes by progressive peristaltic contractions of the body wall. 

 In Spirographs (Sabella) spallanzanii both ends of the tube open to the 

 surface and irrigation waves go in either direction over the animal. The 

 caudal end of the tube of Sabella pavonina, however, is buried, and irriga- 

 tion waves proceed only caudally; water currents emerging from the tail 

 end of the tube percolate to the surface through the overlying sand and 

 mud. The currents subserve feeding and respiration, the latter taking place 

 through the gills and body surface. The tube of Myxicola, another sabellid, 

 is gelatinous and open only at the anterior end; the body does not execute 



Fig. 4.7. Record of Activity of Arenicola marina (7-hour trace) 



The worm was feeding actively from the head shaft with its tail towards the float of 

 the recording apparatus. Activity cycles occur about once per 40 min. Each sharp 

 downward peak is due to tailward excursion of the worm and defaecation, the broad 

 second peak (upward) results from vigorous headward irrigation accompanied by gentle 

 headward creeping. (From Wells, 1949.) 



respiratory movements, and external respiration is largely accomplished 

 through the gills (157, 159). 



Commensal species are frequently provided with water currents by their 

 hosts. Sponges, particularly in shallow warm water, are infested with great 

 numbers of commensals — worms, shrimps, fish and others. These are 

 bathed by the currents which circulate through the cavities of the sponge. 

 Comparable conditions obtain when crustaceans and fish invade the 

 mantle cavities of molluscs, cloaca of holothurians, etc. The tubicolous 

 worms just described — Arenicola, Chaetopterus, Urechis — give shelter to 

 commensal polynoid worms {see Chapter 14). The water currents which 

 the hosts pump through their tubes and burrows meet the respiratory 

 needs of their associates, as well as providing food for the latter. 



Aerial Gills and Accessory Structures. Animals of aquatic origin 

 which have moved up the shore towards land have encountered the prob- 

 lem of adapting their gills for aerial respiration. Essentially aquatic forms 

 which are periodically exposed, notably lamellibranchs and cirripedes, shut 

 their valves and retain a supply of water until the tide returns. During this 

 period of anaerobiosis, metabolism is reduced and a temporary oxygen 

 debt is incurred. The oyster Ostrea virginica, for example, is able to with- 



