. The evolution and function of living purposive matter . ractile fibres or muscle-cells are brought intoplay. Action of thiskind would appear tobe an amplification ofthat which brings thecilia of a volvox intooperation. In jelly-fishes(Medusoids) the nerve-cells form a chainpassing round themargin of the bell, which on the one hand p,g, 20.—Volvox snowing the smaU ciliatedreppivf> imnroooiriTio somatic cells and seven large germ cells, leoeive impressions (Drawn from life bj- J. H. Emerton. See E. made on them through f„ J&e, ° *°° *the various sensory organs, and on the other hand t
. The evolution and function of living purposive matter . ractile fibres or muscle-cells are brought intoplay. Action of thiskind would appear tobe an amplification ofthat which brings thecilia of a volvox intooperation. In jelly-fishes(Medusoids) the nerve-cells form a chainpassing round themargin of the bell, which on the one hand p,g, 20.—Volvox snowing the smaU ciliatedreppivf> imnroooiriTio somatic cells and seven large germ cells, leoeive impressions (Drawn from life bj- J. H. Emerton. See E. made on them through f„ J&e, ° *°° *the various sensory organs, and on the other hand transform this energyinto force, which passes to the animals muscularapparatus and bring it into co-ordinate purposiveaction, thus giving us the first inkling of a centralnervous system. In the next higher class of animals (Echinodermata),which include star-fishes, the ganglionic nerve-cellshave become aggregated into a central nervous organwhich forms a circumaxial ring with radial cords ofnervous matter passing outwards to supply the muscles. 168 THE EVOLUTION OF of the animals limbs with motor impulses, and toreceive impressions passing from the various sensoryorgans to the central nervous organ. The action ofthe purposive elements of the central nervous systemof these animals is conspicuous in the efforts theymake to assume their natural position when they havebeen turned over on their dorsal surface, and in theiraction in getting rid of a harmful source of irritation,which extends so far as to cause them to cast offan offending limb ; which however is soon regenerated(p. 49). In the worms (Annelida) there is a more completeco-ordination of the brain than in the Medusoids, anervous system capable of initiating actions. Thecentral nervous system has assumed a bilateralsymmetrical form; it consists typically of a pairof prceoral (cerebral) ganglia situated in front ofthe mouth, and of a series of post-oral ganglia,arranged segmentally in pairs along the ventral mid-line
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