METAZOA ECHINODERMA. 169 



The preparation (No. 308) shows the nervous system 

 in part. This consists of a ring encircling the esopha- 

 gus, and radial nerves which are the white cords seen in 

 No. 308 running to the end of the arms. 



OPHIUROIDEA. 



The evidence seems to point to the view that the Ophiu- 

 rans have descended from some one of the.more specialized 

 Crinoids - 1 Notwithstanding that many genera retain 

 throughout life the underbasals, basals, and radials of the 

 abactinal area possessed by some Crinoids and by larval 

 Asteroidea, nevertheless peculiar modifications have arisen 

 which place the adult Ophiurans farther from the primi- 

 tive, pentagonal, larval form than the adult Asteroids. 



Generally speaking the radials are developed before the 

 basals and underbasals, and are of large size. We have 

 seen in the specialized Crinoids the tendency toward the 

 increasing development of the radials and the reduction 

 of the b'asals. 



It has been shown by Fewkes 2 and others that the 

 young Ophiuran, like Asterias, is at first bilaterally sym- 

 metrical and that later it takes on the pentagonal form 

 which gradually, with the development of the arms, 

 changes to the modified stellate condition of the adult. 

 The bilateral larva possesses an intestine and anus, but 

 later both disappear, so that the adult is more reduced in 

 this particular than the starfish. 



Ophiopholis aculeata Gray (No. 309), has a circular, 

 sharply defined disc which bears minute spines. The long> 

 rounded, unbranched arms run out directly from the disc 

 and are of about the same size throughout. They are pro- 

 tected by hard plates, dorsal, lateral, and ventral shields,. 



1 Sladen, Quart. Joura. Micr. Sci., XXIV, 1884. 



2 Bull. Mus. Comp. Zool., XIII, no. 4, 1887, p. 107. 



