ORDOVICIAN PERIOD 



23 



STARFISHES 



BRITTLE- 

 STARS 



SEA-URCHINS 



crinoids save these, the mouth of the animal was not exposed 

 to view, but concealed under a disc of little plates stretching 

 across the top of the cup. By this arrangement food brought 

 down by the arms was conveyed to the mouth through 

 small tubes or passages under the disc. In the case of these 

 particular crinoids the mouth was brought into view by dis- 

 placement of the discal plates, and the tubes were superseded 

 by surface channels. The animal could thus take in food 

 more freely, and of larger size ; and in course of time 

 muzzled crinoids all passed away. 



Starfishes, owing to reduced discs, and more sharply 

 defined arms, were now moving about more freely (Palceaster). 

 But their numerous little tube-feet were not so well protected 

 as those of later forms. Brittle-stars attested new develop- 

 ments. Some of these creatures possessed far more flexible 

 rays than the starfishes (Protaster) — owing, no doubt, to the 

 digestive system having become centralised in the disc, and 

 not extending, as with starfishes, into the rays. Echinoids or 

 sea-urchins — unknown in the Cambrian — were certainly now 

 abroad. These Ordovicians — ^harbingers of a multitudinous 

 and widespread race — were about the size of cherries ; 

 and their bristUng body-coverings were more economically 

 constructed than those of any later urchins (Bothriocidaris). 



Here and there in chalky tubes, cemented to the rocks, worms 

 certain worms (Ortonia) swelled the ranks of stationary life — 

 already considerable. 



Among the shelly hosts cemented existence was also 

 noticeable, — some very small lampshells having developed 

 the less laborious method of cUnging to rocks (Crania). 

 The vast majority of lampshells, however, retained their 

 mobility — such as it was — unimpaired. Many of these 

 recalled the forms that lived and died on Cambrian coasts. 

 Those, however, in hinge- worked shells had greatly increased ; 

 and in some cases their shells were strengthened with ribs 

 {Platystrophia, etc.). Their less inventive brethren neverthe- 

 less were still numerous. Certain of the more advanced 

 forms had also developed a Ume framework for the " arms," 

 so that the food-bearing water near the mouth could be 

 better kept in motion {Atrypa, Rhynchonella). In later times 



LAMPSHELLS 



