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

34 CNIDARIAFig. 4.2 (a) The nerve net ofHydra; (b) muscle tails on themusculo-epithelial cells ofaseaanemone,Metridium;(c) transverse sectionof mesentery showing foldingof mesoglea with muscletails, making a longitudinal ‘muscle.’in Hydra, for example, ectodermal cells are continuously producedjust below the mouth region and migrate to the tentacle tips or tothe foot, where they are sloughed off. At a certain distance from thedominant tentacular region, cells can move out and divide to forma bud. This ability to move and differentiate underlies the remarkableregenerative powers of many polyps; an isolated piece of tissuecan often regenerate a polyp with one or more tentacular regions,even after having been put through a sieve, in strong contrast towhat happens with more elaborate animals.The simplicity of cnidarians is thoroughly established by theirdiploblastic constitution, the paucity of cell types, the nature of theonly internal cavity, the lack of organs and the absence of separatemuscle cells and centralised nervous systems. The unique stingingcells (Figure 4.3) make these simple animals viable.4.2 What kinds of Cnidaria are known?There are four classes. The range of structure in cnidarians is shownin Figures 4.4 to 4.8.Anthozoa: anemones and most corals. Polyps with vertical divisions(mesenteries) in the coelenteron (Figure 4.4). No medusaforms.Alcyonaria, polyps with eight mesenteries and typically eightbranched tentacles: sea pens, branching corals, soft corals.Zoantharia, typically with six or twelve mesenteries andvariable numbers of simple tentacles: anemones and ‘true’oceanic reef-building corals.