There is some evidence that the comb jellies (Ctenophores) evolved separately from Their absence of hard parts) and their fossil record is sparse. Jellyfish and sea fansĪre known since Ediacaran fauna of the Precambrian but do not fossilize well (given Has one opening which serves as both their mouth and anus. They have tissues but no organs and their digestive cavity Most cnidarians are marine and all are carnivorous. Ever since then, bilaterally symmetrical animals have been predominant Such as eyes (because then they would need them for all sides of their body).ĭuring the Ediacaran Period, radially symmetrical animals outnumbered bilaterally With radial symmetry are equally receptive on all sides but have few specialized structures Halves, there is more than one plane of symmetry around a central axis. Have the primitive condition of radial symmetry: rather than having right and left (Hydra in the first image sea anenomes in the next two images)Īnimals such as jellyfish which achieve only the tissue level of organization also The following images are of cnidarian tissues: Which includes jellyfish, corals, sea pens, and sea fans). Rise to a number of lineages, including those which led to modern cnidarians (a group Tissues are collections of cellsĪnd cell products that perform a common function. Step for animals was a tissue level of organization. Of animals which resulted in archaeocyathans and modern sponges was the earliest branchįrom the family tree of mammals whose descendants survive today. The most primitive animals are multicellular organisms that lack tissues.
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