NO. 2 



MIDDLE CAMBRIAN HOLOTHURIAXS AND MEDUS.t 



43 



HOLOTHURIANS 



Heretofore the only paleontologic evidence of the Holothurians 

 has been the presence, in rocks of late Paleozoic and post Paleozoic 

 age, of the spicules of those forms having a calcareous subepidermic 

 skeleton. To find, in the Middle Cambrian, representatives of the 

 Actinopoda, both with and without podia, and a form indicating a 

 second order, Paractinopoda, is a great surprise. This estab- 



FiG. 2. — Diagrammatic reconstruction of the imagined primitive Pelmato- 

 zokr ancestor, f After Lankester, 1900, fig. 7, p. 9/) O ^ mouth ; As = anus; 

 ac = right and left anterior portion of ccelom ; rpc and Ipc = right and left 

 posterior portion of coelom ; Ihc ^ left hj'drocoel; sc ^ canal connecting Ihc 

 and ac ; par := parietal canal ; M = dorsal pore ; pi = preoral lobe with nerve 

 center n. 



Fig. 3. — Diagrammatic reconstruction of imagined Pipleiirula ancestor. An- 

 terior end at left of drawing; organs of left side toward observer, and with 

 stronger outline than those of right side. (After Lankester, 1900, lig. i, p. 4.^) 

 O ^ mouth ; As =: anus; ac aright and left anterior portion of coelom; rpc 

 and Ipc = right and left posterior portion of ca4om ; rhc and Ihc =: right and 

 left hydrocoels ; sc = canal connecting Ihc and ac; M = dorsal pore; pi r= 

 preoral lobe with nerve center n. 



lishes the very ancient origin of the Class Holothurioidea and the 

 fact of its great differentiation in Middle Cambrian time. This is 

 particularly true of the free swimming, pelagic form, Eldonia lud- 

 zvigi. 



Among zoologists the theoretically most primitive ancestor of the 



Lankester, Treatise on Zoology, pt. 3, Echinodermata, 1900, p. 9. 



