138 THE BIOLOGY OF MARINE ANIMALS 



respiratory exchange normally effected by gills and skin at low tempera- 

 tures. Absorption of oxygen through the skin amounts to 17-1 c.c./kg an 

 hour at 7-5-8 c C. This level is adequate for metabolic requirements below 

 15°C, but not at higher temperatures (56, 159, 160). 



Gills 



With increase in the size and efficiency of marine invertebrates, gills 

 have been evolved to promote respiratory exchange. These structures are 

 vascularized outgrowths of the body wall, and they either project freely 

 from the body surface (e.g. sedentary polychaetes), or they are enclosed in 

 special chambers (e.g. crustaceans). In either event a circulation of water 

 must be maintained over them in order that they operate efficiently. This 

 is accomplished by ciliary activity, by movement of the gills, by active 

 pumping movements and, on occasion, by locomotory movements. 



Not all gill-like structures are solely respiratory in function, and mere 

 appearance is sometimes deceptive. The elaborate gills of bivalve molluscs, 

 sabellids, protochordates and others are ciliary feeding devices and, as 

 such, are described in Chapter 5 ; in many of these animals the gills are 

 responsible for only a minor proportion of the total respiratory exchange 

 (Table 4.1). In these forms the respiratory function of the gills seems to be 

 a secondary consequence of their large surface area, and food-trapping 

 their primary function. Well-developed circulatory systems are necessary 

 for respiratory gills to achieve maximal efficiency, and this aspect is dis- 

 cussed in Chapter 3. 



In the littoral region animals are encountered in which the gills are 

 adapted for aerial respiration, or are supplemented by other devices. Some 

 Crustacea, indeed, have become predominantly terrestrial in habit. As is 

 usual with creatures living in such transitional environments, the multi- 

 plicity of functional adaptations which they exhibit, in response to un- 

 wonted environmental stresses, demands attention far out of proportion 

 to the numbers of animals concerned. 



Respiratory devices are classified in the following sections on the basis 

 of functional morphology. The arrangement adopted necessitates some 

 unavoidable overlap in considering different physiological aspects of ex- 

 ternal respiratory exchange (Table 4.1). 



Respiratory Aquatic Gills. Polychaetes. Gills exclusively respiratory 

 in function first appear among polychaete annelids. Errant polychaetes 

 such as Nereis and Phyllodoce possess large vascularized parapodia which 

 undoubtedly augment the respiratory surface, as well as acting as loco- 

 motory organs. Specialized branchial structures are found in many 

 families. The lugworm Arenicola bears external branchial tufts on segments 

 of the middle region (Fig. 4.1). Gills on anterior segments of terebellids 

 are dichotomously branched outgrowths of the body wall; in amphic- 

 tenids they are lamelliform and highly differentiated (Fig. 4.2). 



Echinoderms. True gills are found in some echinoderms and supplement 

 respiratory exchange in the tube feet. Those of asteroids are papillate 



