. The origin and evolution of life, on the theory of action, reaction and interaction of energy. lis (Fig. 24) which strikingly suggests one of the existingburrowing sea-cucumbers, Synapta girardil. The character-istic elongated cyhndrical body-form with longitudinal muscle-bands is clearly preserved in the fossil, while around the mouthis a ring of tubercles interpreted by Walcott as calcareousossicles from above which the oral tentacles have been tornaway. A remarkable and problematic mid-Cambrian fossil, Eldonialudwigi (Fig. 24), is regarded by Walcott as a free-swimmingor pelagic animal. I
. The origin and evolution of life, on the theory of action, reaction and interaction of energy. lis (Fig. 24) which strikingly suggests one of the existingburrowing sea-cucumbers, Synapta girardil. The character-istic elongated cyhndrical body-form with longitudinal muscle-bands is clearly preserved in the fossil, while around the mouthis a ring of tubercles interpreted by Walcott as calcareousossicles from above which the oral tentacles have been tornaway. A remarkable and problematic mid-Cambrian fossil, Eldonialudwigi (Fig. 24), is regarded by Walcott as a free-swimmingor pelagic animal. It bears a superficial resemblance to amedusa, or jellyfish, while the lines radiating from a centralring suggest the existence of a water vascular system; but thecylindrical body coiled around the centre shows a spiral intes-tine through its transparent body-wall, and it is therefore con-sidered to be a swimming holothurian, or sea-cucumber, with CAMBRIAN INVERTEBRATES 127 a medusa-like umbrella. The existing holothuroid Pelagothurianatatrix Ludwig, shown at the right, is somewhat analogous,. Fig. 24. Sea-Cucumbers of Cambrian and Recent Seas. Eldonia luiwigioi the mid-Cambrian (after Walcott), regarded as pelagic and somewhatresembling a jellyfish, is thought rather to be a form analogous to Pelagothuria nata-trix, a swimming sea-cucumber, although it shows wide differences. The mouth ofPelagothuria is above the swimming umbrella, the posterior part of the body and theanal opening are below: in the fossil Eldonia both mouth and anus hang below. Mackenzia coslalis, a mid-Cambrian form (after Walcott), strongly resembling the bur-rowing sea-cucumbers, a recent form of which, Synapta girardii, is shown at the pedunculata, another mid-Cambrian form (after Walcott), and a recentrock-clinging form, Pentacta frondosa. although it also displays wide differences of structure. IfEldonia ludwigi proves to be a holothurian, we witness in mid- 128 THE ORIGIN AND EVOLUTION
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