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The higher groups of this kingdom had developed all, or nearly all, the tissues used in building the bodies of higher animals muscular, reproductive, connectile, glandular, nervous, etc. But these are mostly very diffuse. The muscular fibrils of a jelly-fish are mostly isolated or parallel in bands, rarely in compact well-defined bundles.
Between the external transverse and the internal longitudinal layers we often find two muscular layers whose fibres run diagonally. The body is well provided with muscles, but their arrangement is still far from economical or effective. Within the body-wall is the parenchym. This is a spongy mass of connectile tissue in which the other organs are embedded.
But lying in the central axis of the body it furnishes the very best possible attachment for muscles. Around this primitive notochord was a layer of connectile tissue which later gave rise to the vertebræ forming our backbone. The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal marrow running down the back.
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