FOSSIL STAR-FISHES AND SEA-URCHINS 135 



four. In the " brittle-stars " we have the central disc 

 covered with jointed calcareous plates, and the arms 

 defended by four rows of the same. There are sucking 

 feet, however, but the arms are chiefly employed as 



Fig. 112.— Fossil Star-fish allied to 'the modem {Palceasterina primcBVcC) : «, upper 

 surface ; b, lower surface (Ludlow rocks). 



organs of locomotion, in which they are aided by 

 short hooks. The latter take hold of the surface and 

 thus obviate the necessity of sucking-feet. Nature has 

 usually more than one way of 

 meeting a difficulty, and this is a 

 case in point with the progression 

 of the star-fishes. 



Many star-fishes are character- 

 istically deep-sea animals, and per- 

 haps the Echinodermata, to which Fig. ns-^^/^^.w (recent). 

 both star-fishes and sea-urchins belong, range to and 

 continue over deep parts of the ocean-bed, more than 

 any other group of marine animals. Thus, during 

 the deep-sea dredgings of the Challenger^ such genera 

 as Ophionmsmm^ AixhasteVy etc., were dredged up, the 

 latter from more than a mile and a half depth of 



