Page:The Scientific Monthly vol. 3.djvu/332

 3a6 THE SCIENTIFIO MONTHLY

brian sea-shore and tidal conditions closely similar to those of the present time. A specialization toward the spiny or scaly annulat«s at this period is emphasized in euch forms as Canadia spinoea (Fig. 7), a slowly-moving form which shows a development of lateral chetie and overlapping groups of scale-like dorsal spines comparable only to those of the living Aphroditids. An example of this latter family is Polynoe squamata, furnished with dorsal scales. Still other recent forms, such as Palmyra aurifera Savigny, have groups of spinous scales closely re- sembling those of Canadia.

Even the modern freely propelled Chcetognatka have their repre- sentatives in the Mid-Cambrian, for to no other group of invertebrates can Amis!':iina sagittiformi* Walcott (Fig. 8) be referred, BO far as we can judge by its external form. As in the recent Sagitta the body is dirided into head, trunk, and a somewhat fish-like tail. Its single pair of fins of chtetognatii type would perhaps give a clearer afBnily to the genus 8pad-

Fio. 9. J.M.T F.aH iBoypKon,,iu..) 0, TB. «'^- "^^ CODSpicUOUS pair CiUBUAN. Ptvlola nathortU. Mld-CBmbrlBn («ner of tentacles which SUr-

■pond with tbe same number attni obserred Id in modern chxtognaths,

D.ct«i.ti= m.rgin.i altiiough some recent species tmUclei maj hsTH been lott in Peytaia. ihuivub" du.u- .i. t~

show a pair of sensory papillfe mounted on a stalk on either side of the head as in Spadella cepkalopiera Bush. The digestive canal and other digestive organs appear through the thin walls of the body.

A modern group of jelly fishes, the Scyphomedusffi (Fig. 9), is represented by the Middle Cambrian Peytoia nathorsti the elliptical disc of which is seen from below. Although this fossil species is ascribed by Walcott to the group Rhizostoma because of a lack of mar- ginal tentacles, the thirty-two radiating lobes which are so beautifully preserved in the fossil correspond closely with those of the existiog genus Dactylometra. It is possible that the marginal tentacles may have been lost in Peytoia, as so frequently happens in living jelly fishes when in a dying condition.

Fhtla or Fossil Invsbtebrata Protosoa, EeluDodemiata,

Coelenterata, Annalata,

MoHuscoida, Artbropoda,

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