210 THE BIOLOGY OF THE SEA-SHORE 



thing, a tube, affects the function of respiration, are the 

 barnacles and tubicolous worms. Darwin (1851-4), for 

 instance, gives an interesting description of the process of 

 breathing in the former group . Each of the single pair of gills 

 is formed from an inward plicated fold of the membranous 

 sac in which the animal is enveloped. The water is being 

 constantly pumped in and out of this sac by the movements 

 of the opercular valves, which Darwin likens to the heaving 

 of a man's chest. Further, the gills on each side are so 

 closely attached to the point of one of the valves (the tergum), 

 that each time the valve moves the whole gill is caused to 

 open out as if moved by a lever. 



Tubeworms, as we have already noticed, breathe, to 

 some extent at least, by means of the terminal tentacles or 

 " gills " which are left protruding from the open end of 

 the tube. In addition, there are the parapodial cirri which 

 are present in each segment and are sometimes very con- 

 spicuously developed {e.g. Nerine, Cirratuliis). While the 

 direct respiratory value of these is no doubt small, it should 

 be taken into consideration that they are covered with cilia 

 which ensure a plentiful circulation of water round the 

 body itself. Indirectly, therefore, the cirri may have 

 considerable respiratory value. In worms like Sabella, 

 aeration of the body in the tube is obtained by the alternate 

 protrusion and retraction of the " branchial crown " which 

 is here supported by a skeleton. Other tubicolous forms 

 obtain a more active circulation of water about their bodies 

 by performing vigorous undulatory movements within their 

 tubes. Burrowing worms exhibit analogous behaviour. 

 According to Bohn (1901), the lug-worm, for instance, does 

 not always frequent a U-shaped burrow, as is generally 

 stated, but is often found occupying a vertical position 

 (cf. Gamble and Ashworth's " Laminarian " form), head 

 downwards, and then behaves exactly like a Pectinaria in 

 its tube, causing the water, by means of dilating successive 

 segments of the body, to flow either up (inverse current) or 

 down (direct current). When the animal is submerged the 

 direct current is the one employed (cf. Corysies, p. 213), 



