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

WHY DO WE REGARD CNIDARIA AS SIMPLE?33Fig. 4.1 Cnidaria: (a) polypand (b) medusa forms;(c) longitudinal section of Hydra toshow cell types scattered inthe two layers.all other multicellular animals (except sponges), which are ‘triploblastic’,having three cell layers. Figure 4.1c shows that cells of thesame type are not arranged together in either layer (the slightlyconfusing description of Cnidaria as having ‘tissue grade’ organisationemphasises that there is no aggregation of tissues to makeorgans). There is no brain or central nervous system, but a networkof multipolar nerve cells conducts slowly in all directions(Figure 4.2a). There are no separate muscles but ‘musculo-epithelial’cells of the ectoderm and endoderm are drawn out at the cell-baseinto contractile muscle tails that extend up and down or aroundthe animal in the mesoglea. These muscle tails form sheets thatmay then be folded to make compact structures. This is a uniquemethod of making solid muscles (Figure 4.2b,c).As all cells in both layers are directly in contact with environmentalwater, either on the outside of the animal or in thecoelenteron, there are no special structures for respiration andexcretion, nor is there a transport system (apart from sea-waterchannels in some large jellyfish).Cell movement is a further distinctive property of Cnidaria.Not only are there migratory stem cells (called interstitial cells)that give rise to the nematocysts, nerve cells and gonads, but alsoapparently more differentiated cells may continuously undergo celldivision and move. This capacity is most marked in hydrozoan polyps;