For those of you who don’t want to turn your computer monitor upside down to read the answer to question #3, “Do jellyfish have brains?” the answer given is, “Nope, they have a network of nerves but no central location to it.”
Which is only partly correct.
It is not true that jellyfish have no central nervous systems. They have an unusual nervous system, because jellyfish are not bilaterally symmetrical – that is, they don’t have a left side and a right side. So while most invertebrates have a chain of ganglia lying down the middle of the body, with one very large one at the front end (the brain), jellyfish don’t.
Instead, jellyfish have a ring nervous system, located along the margin of the bell. There is definitely a concentration of neurons in that location (although it contains relatively few neurons compared to other animals). Plus, those neurons do serve as an active relay and processing station for sensory and motor activity. Those are two of the main things that central nervous systems do, so there seems to be no good reason to deny that jellyfish have a central nervous system. (Picture from Mackie and Meech, 2000.)Mackie and Meech (1995) credit the initial discovery of nerve rings to Passano in 1965, so we’ve known about the existence of jellyfish central nervous systems for at least a few decades.
It is true that jellyfish have no brains. But that may not be so bad. Mackie and Meech (2000) wrote:
(T)he lack of a brain is... an adaptation to radial symmetry rather than an indication of primitiveness(.)
Take that, bilateral bigots!
Reference
Mackie G, Meech R. 1995. Central circuitry in the jellyfish Aglantha. I: The relay system. The Journal of Experimental Biology 198: 2261-2270. http://jeb.biologists.org/cgi/content/abstract/198/11/2261
Mackie G, Meech R. 1995. Central circuitry in the jellyfish Aglantha. II: The ring giant and carrier systems. The Journal of Experimental Biology 198: 2271-2278. http://jeb.biologists.org/cgi/content/abstract/198/11/2271
Mackie G, Meech R. 1995. Central circuitry in the jellyfish Aglantha. III: The rootlet and pacemaker systems The Journal of Experimental Biology 203: 1797-1807. http://jeb.biologists.org/cgi/content/abstract/203/12/1797
Jellyfish photo: User mayhem on Flickr.
In higher order animals the nervous system consists of two divisions, the central nervous system and the autonomous nervous system. It would probably be more accurate to describe the jellyfish's system as autonomous or possibly as a separate third type since it does not use ganglia to process stimuli as seen in the slightly more advanved annelids.
ReplyDeleteThat the ring structure of the jellyfish central nervous system is essentially equivalent to other invertebrate ganglia is the argument, though. And if you grant that the ring around the bell is the central nervous system, it follows that the parts of the nervous system that lie outside is are what you've termed the autonomous nervous system (did you mean autonomic?).
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ReplyDeleteThe nervous ring coordinates the pulsation of the whole muscle ring of the bell. More muscles = More enervation. But I've not seen yet any good evidence that in the ring is processed any information. Such kind of concentrated fascicles exists even in human organism. For instance in the heart. The nervous fascicles assures autonomous heartbeat. But there is no information processed inside it.
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