HARDWICKE'S SCIENCE-GOSSIP. 



iS7 



and the Echini on the other. The Cushion-stars 

 (Goniasters), run very near to the Cake-urchins or 

 Clypeasters, although the former are star-fishes and 

 the latter sea-urchins, and perhaps both these touch 

 as nearly as any of their class to the Cystideans, 

 Pentremites, and Marsupites. 



Both star-fishes and sea-urchins are, geologically 



been in existence throughout all the silent revolu- 

 tions, physical and biological, which have so often 

 taken place on the surface of the globe, and our 

 modern star-fishes are as lineal and directly uninter- 

 rupted descendants of these early Cambrian fossil 

 forms, as mankind are from their " first parents." 

 The upper part of the skin of such star-fishes as the 



Fig. 130. — Separate hooks of "Brittle Star' 

 Tosula), much magnified. 



speaking, exceedingly ancient. With the exception 

 of certain Brachiopoda, we know of no other group of 

 animals which have maintained their peculiar shapes 

 for a longer time than the star-fishes. As far back as 

 the Cambrian period we find two well-differentiated 

 orders in existence, one represented by the modern 

 "five-fingers" (Uraster) and the other by the brittle- 

 stars (Ophiura). Evidently these two types have 



Fig. 133.— Fossil Star-fish (PaUrocotna Marstotii), Lower 

 Ludlow Rocks. 



"five-fingers" {Uraster nibens) is thickened and 

 roughened and strengthened by the presence of grains 

 or irregular spicules of carbonate of lime. If each 

 of these grains had gone on increasing in size by 

 addition to its margin, they would have grown until 

 they touched each other, but would not have fused, 

 and then we should have had regular plates instead of 

 grains, and the whole body would have been covered 

 by a kind of tessellated pavement. This is exactly 



