Cnidaria, such as jelly fish and sea anemones are radial symmetric en feature a simple body plan. One notable aspect is the lack of a brain. Hence, there is no central guiding structure, no mind or will if you like. Nonetheless, they are known to be predatory (such as the sea anemones), and are quite effective in defending themselves as well (such as stinging jelly fish). How does a diffuse nervous system allow Cnidaria to feed and defend themselves?

Sea anemone devouring a crab:

Sea Anemone Devouring Crab

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    $\begingroup$ Sea anemones CAN move around. $\endgroup$
    – March Ho
    Jan 22 '15 at 8:26

Sea anemones are members of the phylum of Cnidaria. Distinguishing features of this phylum are radial symmetry and the presence of cnidocytes, or stinging cells (like the ones found in jelly fish). These cells have a mechanical trigger and if activated a subcellular harpoon is fired and paralyzing and painful toxin is injected. A cnidocyte diagram is shown below (from Wikipedia):


The Cnidarian nervous system is relatively simple indeed. The Cnidarian nervous system is decentralized and diffuse, and it basically consists of reflex arches. It is referred to as a "nerve net" as shown in the following schematic of a sea-anemone's nervous system (picture from Florida Int. University):

Nerve net

The presence of cnidocytes and a mesh nervous system basically answer your question:

1) Feeding: Whenever a prey touches the sting cells, it gets harpooned and paralyzed. The mechano-sensory cells sense touch, and drive nervous cells in the mesh. The nervous cells activate contractile epithelial cells in the arms of the anemone that push the prey inside the mouth. Then digestion takes place and the mouth serves as anus once digestion is complete.

2) Defense: the cnidocytes operate by mechanical triggering and no nerve cells are necessary.

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    $\begingroup$ Lol, its nerve cells not nervous cells! Beautiful answer though $\endgroup$
    – One Face
    Jan 23 '15 at 2:42
  • $\begingroup$ Are all toxins from Cnderians actually powerful for every creature? $\endgroup$
    – vajra78
    Jan 24 '15 at 2:37
  • $\begingroup$ @vajra78: no, e.g. some fishes are resistant and actually live in anemones as a safe home. $\endgroup$
    – AliceD
    Jan 24 '15 at 2:41

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