126 THE ORIGIN AND EVOLUTION OF LIFE 



not only by their habit of hiding in crevices, but by their 

 leathery epidermis, in which are scattered a number of cal- 

 careous plates, as among certain members of the modern eden- 

 tate mammals. Fossils of this group have been known here- 

 tofore only through scattered spicules and calcareous plates 

 dating back no earlier than Carboniferous times (Goodrich); 

 therefore Walcott's holothurian material from the Cambrian 

 constitutes new records for invertebrate palaeontology, not 

 only for the preservation of the soft parts, but for the great 

 antiquity of these Cambrian strata. In Louisella pedunculata 

 (Fig. 24) we observe the preservation of a double row of tube- 

 feet, and the indication at the top of oral tentacles around the 

 mouth like those of the modern Elpidiidae. A typical rock- 

 clinging holothurian is the recent Pentacta frondosa. 



Besides these sessile, rock-clinging forms, the adaptive 

 radiation of the holothurians developed burrowing or fossorial 

 types, an example of which is the mid-Cambrian Mackenzia 

 costalis (Fig. 24) which strikingly suggests one of the existing 

 burrowing sea-cucumbers, Synapta girardil. The character- 

 istic elongated cyhndrical body-form with longitudinal muscle- 

 bands is clearly preserved in the fossil, while around the mouth 

 is a ring of tubercles interpreted by Walcott as calcareous 

 ossicles from above which the oral tentacles have been torn 

 away. 



A remarkable and problematic mid-Cambrian fossil, Eldonia 

 ludwigi (Fig. 24), is regarded by Walcott as a free-swimming 

 or pelagic animal. It bears a superficial resemblance to a 

 medusa, or jellyfish, while the lines radiating from a central 

 ring suggest the existence of a water vascular system; but the 

 cylindrical 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 



