CAMBRIAN INVERTEBRATES 



127 



a medusa-like umbrella. The existing holothuroid Pelagothuria 

 natatrix 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 somewhat 

 resembling 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 of 

 Pelagothuria is above the swimming umbrella, the posterior part of the body and the 

 anal 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 right. 

 Loiiisella pedunculata, another mid-Cambrian form (after Walcott), and a recent 

 rock-clinging form, Pentacta frondosa. 



although it also displays wide differences of structure. If 

 Eldonia ludwigi proves to be a holothurian, we witness in mid- 



