RESPIRATION 137 



Small organisms and members of primitive phyla secure adequate oxygen 

 by diffusion, without the provision of special respiratory mechanisms. 

 Such include the majority of protozoa, sponges, coelenterates, turbel- 

 larians, small nemertines, chaetognaths, the smaller annelids, bryozoans, 

 eggs and embryos. This process is adequate to secure respiratory exchanges 

 across short distances, not exceeding 1 mm. Even among these animals, 

 however, we find mechanisms for moving the external medium and so 

 renewing the oxygen supply of the water layer lying immediately over the 

 surface. Examples are flagellar activity in sponges, and ciliary activity in 

 coelenterates, turbellarians, annelids and echinoderms. 



In higher animals, increased oxygen requirements have been met by the 

 evolution of special respiratory structures and mechanisms. These include 

 respiratory organs and appendages; mechanisms for renewing the external 

 medium, whether water or air, over or within these structures; circulation 

 of internal fluids, and respiratory pigments for increasing the oxygen- 

 carrying capacity of the blood. Nevertheless, in the final analysis, gaseous 

 exchange across the external respiratory surface, and between the body 

 fluids and tissue cells, is carried out by diffusion and these distances are 

 usually very short. In gills of decapod Crustacea, for example, the span 

 between the haemolymph and the outside of the gill is narrow, between 

 3-5/u. The internal medium, in turn, bathes all the tissues of the body 

 (78, 86, 145). 



RESPIRATORY MECHANISMS 



In aquatic animals external respiration takes place across the integument, 

 gills and in specialized portions of the alimentary canal. Aerial respiration 

 takes place across gill surfaces, in lungs and in tracheae. The latter are 

 characteristic of insects, few of which have invaded the sea. 



Integument 



Some degree of respiratory exchange across the general integument 

 takes place in nearly all marine animals, from the simplest metazoans to 

 fish. For cutaneous respiration to be effective in aquatic animals there 

 must be a current of water over the skin. Respiration is entirely cutaneous 

 in many lower animals — turbellarians, nemertines, many annelids, echiu- 

 roids, sipunculoids, chaetognaths, small arthropods, embryos and young 

 larvae — all of which respire through the general body surface. Even when 

 gills are present in aquatic animals, much gaseous exchange still takes 

 place through the general integument. Quantitative information dealing 

 with partition of respiratory function between gills and general integument 

 is scanty. In specimens of sabellid worms (Bispira, Sabella), from which the 

 branchial crowns have been removed, respiration through the general body 

 surface continues at around 36 % of levels for the whole animal. Similar 

 experiments on Myxicola indicate that cutaneous respiration accounts for 

 about one-half of the oxygen requirements of the abdomen. Respiration 

 through the skin of the eel Anguilla can provide 60 % of the total external 



M.A. — 5* 



