That only one of the four gastrial pores was retained. Perhaps the biggest leap comes next, and weĬan choose what to believe to a certain extent. These would initially serveĪs outlets for indigestible particles i.e. Pores is likely to be four (two implies bilaterality, three impliesĪ funny symmetry that we don't see, and we do find four outletsįrom the gut in some coelenterates). The beast is radially symmetrical the minimum number of such Openings, gastrial pores, appeared around the apical organ. Let us suppose that a series of secondary The least efficient feature of this animal Radially symmetrical) between this ring nerve and the apical The mouth and probably several links (because this animal is If we believe in an animal like that we canįill in a few more probabilities too, an apical sensory organĪs before, a ring nerve below the archaeotroch and thus around Naturally goes around the opening of the gut. The downstream ciliary collecting band or archaeotroch So lets modify our planktonic organism (OVERHEADĪGAIN). A downstream band would give better swimming,Īllow an increase in body size and waft food particles towards Usually occur) are classified according to function as upstream The story isn't quite as simple as it mightīe, however, because this sort of modification seems to haveĪrisen twice, and the compound cilia function differently inĮach case (OVERHEAD). Compound cilia (more than 1 per cell), have greater To be the formation of a band of compound cilia, which are moreĮffective. The next advance we see, in the plankton appears Cilia are small, and have a limited power output. Was, and still is, limited by the effectiveness of the one cilium The size of both hydroids and planktonic animals Or as a specialisation of a free living planktonic (swimming This would again be causedīy a change of lifestyle, a specialisation to a potentially betterĮnvironment, and we already have two possible points at whichĪn animal may have settled on the bottom, either as a blastaea, Upwards and the apical organ stuck in the mud - no great lossīecause it isn't going anywhere. Radially symmetrical, with the opening of the gut pointing We find that we have made a cnidarian coelenterate (OVERHEAD) Now if we simply flip this over through 180o All cells would retain a cilium, which would be used for The hole in the middle of the ball of cells and we might evenįind an inpushing, a cavity, a gut lined with digestive and absorbingĬells. The animal formed a dent with the new cells pushing upwards into Make it larger, and it is possible that to fit in more cells Local multiplication of cells in this area would LocalisedĬell division would perhaps be required to ensure an adequateįood supply. In this position we might find digestive cells. Which didn't get brushed aside as the organism rolled through To leeward and offered the opportunity of retaining food particles The side opposite the apical organ was now To happen was a preferred direction of swimming, with sensoryĬells collected at one particular place - lets call it the apical No special preference (OVERHEAD): probably the first thing How might aįree swimming blasteae cope better with swimming in the sea?Ī blastaea would roll through the water with ![]() Organisms live in water rather than ion the mud. Lets look now at an alternative scenario: many To the bottom, and that contact with the mud might set off aĬellular reaction leading to the defining of a ventral pole -Ī bottom end. We suggested that the organisation of thisĪnimal might be affected by its environment - that it might sink This will allow cells which don't feed to survive. ![]() This animal has a nice isolated cavity, the blastocoel cut offįrom the outside world and perhaps serving as a means of passageįor organic molecules from cell to cell without getting lost. (OVERHEAD) but probably with more cellular specialisation. Organisms blastaea, looking rather like a colony of choanoflagellates Last time we looked at the first metazoansĪnd the way in which we think they initially formed a hollowīall of cells with a cavity inside: we called these hypothetical ![]() The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences. These pages have been left in this location as a service to the numerous websites around the world which link to this content. Evolutionary Developmental Biology Faculty of Biological Sciences, University of Leeds
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