ORIGIN AND EVOLUTION OF LIFE 323
pidae, the typical species of which, Molaria spinifem Walcott, may be compared vith that "living fossil" the hoi^eshoe crab {Limvlua polyphemva), its nearest modern relative, which is believed to be not 80 closely related to the phyllopod crustaceans as would at first appear, but rather to the Araclmida through the Eurypterids and scorpions. Molaria and lAmiilva are strikingly similar in their cephalic shield, segmentation, and telson; but the latter shows an advance upon the earlier type in the coalescence of the abdominal segments into a single abdominal shield-plate. The trilobate character of the cephalic shield in Molaria is an indication of its trilobite affinities; hence we appar- ently have good reason to refer both the merostomes and phyllopods to an ancestral trilobite stock.
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��Another mode of defence is presented by the sessile, rock-clinging sea-cucumbers (Holothuroidea) protected by their leathery epidermis in -which are scattered a number of calcareous plates, as among certain members of the modern edentate miammals. Fossils of this group have been known heretofore only through scattered spicules and calcareous plates dating back no earlier than Carboniferous times (Goodrich); therefore Walcotfs holothurian material from the Cambrian consti- tutes new records for invertebrate paleontology, not only for the pres- ervation of the soft parts, but for the great antiquity of these Cam- brian strata. In Louisella peduncvJata {Fig. 6) we observe the pres- ervation of a double row of tube-feet, and the indication at the top of oral tentacles around the mouth like those of the modern Elpidiidfe. A typical rock-clinging holothurian is the recent Peniacia frondosa.
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