An Introduction to the Invertebrates, Second Edition - tiera.ru

TRANSPORT SYSTEMS111lobes with a large surface area and internal blood supply. Tube-livingpolychaetes may need separate gills if the tentacles collect foodfrom sand.Respiratory pigments are now necessary (see Box 9.3), either inthe blood to increase its oxygen carrying capacity or in tissues tofacilitate diffusion. Haemoglobin is the commonest respiratorypigment, occurring in a very wide range of animals. It occurs inmany annelids, but so also do two other rare iron-based pigments,haemerythrin and chlorocruorin, and the copper-based haemocyanin:no other phylum has so great a range.Box 9.3Transpor t systemsThe need for a transport systemDiffusionDiffusion, the movement of a substance from a higher to a lower concentrationof that substance, is not a sufficient means of communicationbetween the parts of any but the smallest animals. Planarians, for example, relyon diffusion for their oxygen supply and typically consume about 0.1 to 0.2 mloxygen per gram per hour at 15 °C. Calculations based on the rate ofdiffusion suggests that planarians should be no more than 0.5 mm thick.[They do not all obey. At this point biology takes over from physics: mostplanarians respire anaerobically part of the time, and much of the centreof the worm is nottissue but food in the gut.The general point, however, is valid.]The transport or ‘vascular’ systemThe commonest transport system is blood, a fluid tissue (i.e. it contains cells)either in closed vessels or in a haemocoelic cavity. Its function is to transportrespiratory gases, food, excretory matter, hormones and many othersubstances. The assumption that the earliest blood systems evolved primarilyfor oxygen transport may be mistaken: nemertines, for example (seeChapter 7), have a blood system lying deep in the body and not mainly concernedwith oxygen transport. Nemertines, unlike planarians, have a gutopening at both ends with one-way movement of food; this at once demandsa transport systemif foodis to reach all parts ofthe body.Even in planarians, where the mouth is the only opening, food wouldnot reach all tissues but for the considerable branching of the gut.Transport of oxygen and carbon dioxideRespiratory pigmentsPigments that combine reversibly with oxygen are in most animals a necessaryaid to respiration. By far the commonest is haemoglobin (Hb), found at least