OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 37 



The arm segments are indicated by a succession of rings, alter- 

 nately slightly projecting and slightly depressed, which appear on 

 the dorsal surface and on the sides of the arms. The projecting por- 

 tions show for the most part two fairly regular rows of little 

 rounded and projecting nipples which serve for the insertion of the 

 hook. Through almost the entire length of the arms these hooks are 

 for the most part torn away, and it is easy to make out in the middle 

 of the nipples the characteristic small central depression. Between 

 these two ranges of nipples and outside of them the surface of the 

 segment is covered by little plates with a swollen surface, which are 

 somewhat unequal, polygonal or rounded in outline, and which pass 

 over into the very much lower and imbricated plates covering the 

 depressed regions of the arms. These last are narrow, vertically 

 elongated, and form several irregular rows. 



The hooks borne by the nipples measure 0.4 mm. in length. Their 

 form is very simple, and they do not show any accessory tooth below 

 their very strongly recurved and pointed tip. In figure 5 I give a 

 photograph of one of them. 



The double rows of hooks follow each other with very great regu- 

 larity, except on the four or five first arm segments, where thefy are 

 not as yet very regularly arranged, and they continue throughout 

 the whole length of the arm. They may be very easily distinguished 

 on the photographs which I give of arms viewed laterally and at 

 different parts of their length (figs. 3, 6). 



The arm spines or tentacle scales first appear on the pores of the 

 second pair, which show sometimes a single, sometimes two spines; 

 the pores of the third pair are usually provided with three spines; 

 those of the fourth pair have three and sometimes four, and beyond 

 the fifth pair the number is four ; this number is maintained through- 

 out the greater part of the length of the arms. These spines are large, 

 short, thick, and conical, with the point blunted ; the three first are 

 subequal, and their length almost equals two-thirds the length of 

 the segment; the fourth (dorsal) spine is a little smaller and a little 

 more slender than the others. Their surface is somewhat rugose and, 

 under the microscope, it shows only very fine, short, and crowded 

 spinules. The four spines are very close together, forming a short 

 transverse row, which lies for the most part on the ventral surface 

 of the arms. 



The number of the spines diminishes only near the tip of the 

 arm, and they finally become reduced to two, at the same time becom- 

 ing relatively more slender, but without ever transforming into 

 hooks. 



The color of the specimen in alcohol is uniform gray. 



Affinities and distinctive features. — This ophiuran appears to me 

 to fall Avithin the genus Astrothamnus which was erected by Matsu- 



