506 THE MICROSCOPE. 



aura, a tail) ; the body forms a roundish or somewhat 

 pentagonal disc, furnished with five long simple arms, 

 which have no furrow for the protrusion of the ambulacra. 

 Ophiuridea are exceedingly plentiful in all 

 our seas, and their remains occur in all 

 the more recent marine strata of the 

 earth's crust. They are more commonly 

 called Sand Stars, or Brittle Stars. 



"However much the faunas of the 

 various geologic periods may have differed 

 from each other, or from the fauna which 

 now exists, in their aspect and character, 

 they were all, if I may so speak, equally 

 underlaid by the great leading ideas which 

 still constitute the master types of animal 

 life. And these leading ideas are four in 

 number. First, there is the star-like type! 

 of life, life embodied in a form that, as 

 in the corals, the sea anemones, the sea 

 urchins, and the star fishes, radiates out- 

 wards from a centre ; second, there is 

 the articulated type of life, life embodied 

 in a form composed, as in the worms, 

 crustaceans, and insects, of a series of 

 rings united by their edges, but more or 

 less moveable on each other ; third, there 

 is the bilateral or molhcscan type of life, 

 Fig. 24i. Encrinus, life embodied in a form in which 

 there is a duality of corresponding parts, 

 ranged, as in the cuttle fishes, the claws, and the snails, 

 on the sides of a central axis or plane ; and fourth there is 

 the vertebrate type of life life embodied in a form in 

 which an internal skeleton is built up into two cavities 

 placed the one over the other, the upper for the reception 

 of the nervous centres, central and spinal, the lower for 

 the lodgment of the respiratory, circulatory, and digestive 

 organs. Such have been the four central ideas of the 

 faunas of every succeeding generation, except perhaps the 

 earliest of all, that of the Lower Silurian System, in which, 

 so far as is yet known, only three of the number existed, 

 the radiated, articulated, and molluscan ideas or types. 



