76 STAR-FISHES, SEA-URCHINS, ETC. 



5, Fig. 2) is of a reddish-brown color, and com- 

 pletely covered over with minute silken bristles. 

 These, on the death of the animal, are rapidly re- 

 moved, and then the flat disk can be plainly seen 

 to be made up of a large number of closely-fitting 

 polygonal plates, arranged in twenty series around 

 the circumference. The petals of the central star, 

 which now becomes visible, will be found to be 

 made up of transverse slits, and if the direction of 

 the petals is followed to the border of the disk, it 

 will be seen that the slits are continued by pores. 

 Through these slits and pores, which occupy five 

 pairs of plates, the ti^be-feet, similar in character to 

 those of the star-fish, pass out, and hence define 

 ambulacral zones. The intermediate five pairs of 

 plates, from which pores are absent, will then be 

 the interambulacral areas. Now, imagine the arms 

 of a star-fish turned over the back of the animal so 

 as to have the tips meet, the animal then flattened 

 out, and the sides of the arms so expanded as to 

 close in the interspaces: you would then have a 

 construction much as in the sand-dollar and in other 

 sea-urchins. This appears really to be the relation, 

 but just how the diverging modification has been 

 brought about still remains to be determined. The 

 eye-specks, which in the star-fish are placed at the 

 extremities of the arms, are in the urchin situated 

 centrally on top, or just where we should expect to 

 find them on the assumption above stated. 



Alternating with the eye-specks are the ovarian 

 apertures, through which the eggs are passed out 

 from the body. In the cluster of small plates which 



