THE STELLEROIDEA 



261 



two families the Ophiuridae, including the forms with simple 

 arms, and the Astrophytidae, the members of which have branched 

 arms. Raised to sub-orders and divided into families, these two 

 divisions have long survived. In 1892 Bell (3) proposed a classi- 

 fication into three orders, to which a fourth has been added to 

 include some fossil forms (Gregory, 15). 



The first important contributions to the anatomy of the 

 Ophiuroids were an account of the structure of the Euryaleae by 

 L. Agassiz, and of the skeleton of various genera by Miiller 

 ( 1 854). Further researches have been carried out by Cue'not (1888) 

 and Hamann (1889). The two authors, however, to whom we are 

 most indebted are Lyman, who, in a long series of papers, has 

 described the skeletal structures, and Ludwig, whose masterly 



FIG. XIII. 



Ophinpholis aculeata, abactiual surface. 



memoirs gave the first detailed account of the visceral anatomy. 

 Genera of exceptional interest have been described by Bell, 

 Ludwig, Simroth, and Sladen, to whom we owe memoirs 

 respectively, on Ophioteresis, Trichaster, Ophiadis, and Astrophiura. 

 The development was first studied by Miiller and Krohn (1851); 

 Ludwig in several memoirs, and Apostolides (1882), Russo (1891), 

 Cue'not (1892), and Bury, have studied additional forms with modern 

 methods; while MacBride, in 1896, has shaken, if not destroyed, 

 faith in the theory of the homology of the "calycinal" plates 

 (p. 14) as taught by Lov4n, Carpenter, and Sladen. 



The body of an Ophiuroid, like that of an Asteroid, consists of 

 a central disc, from which radiate several (generally five) arms (Fig. 

 XIII.). The disc, however, is not formed, as it is in Asteroids, by 



