70 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery 

 VIII. 



Class OPHIUROIDEA. 



Table-case 

 29. 



Upright 

 ease A3. 



Table-case 

 29. 



Wall-case 

 17 a. 



The 



living 



Brittle-stars, Sand-stars and Basket-fish are 

 separated from the star-fishes as a Class, because the arms 

 are sharply marked off from the central disc, and have the 

 grooves covered over by plates, and the flooring-plates of the 

 grooves fused into a series of ossicles (little bones) like 

 vertebrae, worked on one another by powerful muscles. 

 Thus these arms can serve as limbs for locomotion ; and the 

 podia, not being needed for that purpose, usually serve only 

 for respiration and touch. As a further result of this 

 development, the arms no longer contain processes from 

 the digestive and reproductive systems as they do in 

 star-fish. In the Basket-fish the arms may branch, and 

 are used for coiling round the stems of other animals or 

 plants. 



The Palaeozoic Ophiuroids do not show all these 

 points of distinction from Asteroids ; in many of them the 

 arm-groove is not completely 

 closed, and its flooring-plates 

 are not yet fused into vertebrae. 

 Species found in the Ordovician 

 rocks of Bohemia are still more 

 like Asteroids than any here 

 exhibited. We begin with British 

 Wenlockian forms, such as Lap- 

 vjorthura (Fig. 33) and Protaster 

 from the Lower Ludlow shales. 

 A slightly more advanced type 

 is the little Sympterura from the 

 Lower Devonian of Cornwall. 

 The Ophiuroids of this age, 

 must, however, be studied in the 

 Stiirtz Collection from Bunden- 

 bach, where explanatory labels 



Fig. 33.— a Palaeozoic Ophiuroid, 

 Lapivorthura Miltoni. Lower 

 Ludlow shales. Shows the 

 mouth-frame in the centre of 

 the round hody. Between two 

 of the rays is the madreporite, 

 of which an enlarged figure is 

 given. 



Among the British Wenlockian Ophiuroidea the most 

 remarkable are Euclaclia and its allies ; for in them the arms 

 do not extend beyond the disc, but to make up for this the 

 few podia within the disc limits are of great size, and have a 

 flexible armour of small plates. 



From Carboniferous to Trias there are no British 

 Ophiuroids, but on the lowest slope of Wall-case 17a may 

 be seen Onycliaster, from the Carboniferous rocks of Indiana, 



