RESPIRATION IN SHORE ANIMALS 211 



the water then fihering through the acid faeces or through 

 the sand held by the parapodial bristles, and losing in conse- 

 quence any possible toxicity due to its being alkaline. At 

 low water the current is inverse or nil. 



Respiration in Relation to Burrowing. — The habit 

 of burrowing is particularly useful on the sea-shore since 

 it not only secures to an animal a quantity of food in a 

 conveniently fine condition, but also provides a satisfactory 

 response to several urgent problems such as the need of 

 escaping observation by enemies, and of avoiding the conse- 

 quences of wave action and of the retreat of the tide. It is 

 therefore not surprising to find the soil of the tidal zone 

 supporting not only a large permanent population, but also 

 affording a sanctuary to other more active animals when this 

 area is uncovered. The most characteristic burrowing 

 forms are " worms " (including Nemerteans, Oligochaets, 

 and Polychaets), crustaceans, and bivalve molluscs. Two 

 particularly common burrowing Polychaets are the lug- 

 worm {Arenicola) and Cirratulus. The former has its 

 respiratory surface augmented by a series of bushy gills 

 which, in view of their delicate nature and exposed situation, 

 one cannot but feel are ill suited to a burrowing animal. 

 On the other hand, according to the observations of one of 

 the writers, the respiratory arrangements in Cirratulus 

 appear to be remarkably well adapted to a subterranean 

 existence. The presence of this worm in its natural habitat 

 is denoted by a group of deHcate bright-red processes or 

 cirri lying on the surface of the sand or gently waving in the 

 water of a pool. These cirri, which are remarkably long 

 and elastic, occur in paired clusters a short distance behind 

 the anterior end and also in single pairs on each segment. 

 When the worm burrows, the filaments are left protruding 

 in the water at the surface and they almost certainly serve to 

 keep the blood aerated while the owner is finding its food 

 in the distinctly foul mud which it prefers. 



The danger of asphyxiation in the rich mud of the sea- 

 shore is by no means a remote one and, in fact, in the course 

 of a hot summer may become acute. Thus, Bohn {op. cit.) 



