Page:The Scientific Monthly vol. 3.djvu/331

 ORIGIN AND EVOLUTION OF LIFE

��a water vascular system; but the cylindrical body coiled around the center shows a spiral intestine through ita transparent body-wall, and it is therefore considered to be a swimming holothurian or sea- cucumber with a medusa-hke umbrella. The existing holo- thuroid Pelagoihuria natatrix Ludwig, shown at the right, is somewhat analogous although it also displays wide differences of structure. If Eldonia lud- teigi proves to be a holothurian we witness in Mid-Cambrian strata members of this order differentiated into at least three widely distinct families. The worms, including swim- ming and burrowing annu- lates, are represented in the Burgess fauna by a very large number of specimens, com-

��anadia ipino

��(nfter

��tt) with c

��lapping groupfl of reiiEiublIng ItiMe of the living Aphroditidiz, luch as Polunoe iijuamaia Worthenella Cambric a worm ol Mld-CHmbrian time* (After Walcatt) campared with XereU vtraui and Arabella opalina, revmt murine worms.

��Pic, 8. Peeelt SwiimiNO Cb«tc MATHS. AmUtu/la aaglltltoTmle. a M CimbrlaD form (after Wslcott), has bodr dMded Into head, Irunk aad ti like th« recent Sagilta. ai leen in ffarHneri,

��prising nineteen species, distributed through eleven genera and six fami- lies. Most of these are of the order Polych£eta, as, for example, Worthen- ella camhria, in which the bead is armed with tentacles, while the seg- mented body and the continuous series of bilobed parapodia are very clear. When compared with such typical hving polychtetes as Nereis i-irens and Arabella opalina (Fig. 7), , we have clear proof of the modern relationships of these Mid-Cam- brian species, as well as of Cam-

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