No. 19.] 



ECHINODERMS OF CONNECTICUT. 



93 



here surround a single central plate, however, instead of the 

 intestinal opening. 



The ambulacral areas are plainly marked on the aboral sur- 

 face of both species by the rows of respiratory tube-feet which 

 are arranged symmetrically about the central plate. The figure 

 thus formed has the general shape of a five-petaled flower 

 (Plates XXVII and XXVIII), and the two groups of each ambu- 

 lacral area are together called a " petal." The petals are even 

 more clearly indicated by the rows of double pores on the surface 

 of the disk after the removal of the spines (Fig. 14). 



nhr. 



FIG. 14. Echinarachnius par-ma. Outline 

 of plates on aboral surface; amb, ambulacral 

 areas; inter, interambulacral areas; a, position 

 of intestinal opening. The four genital pores 

 are shown near the center of the disk. 



In contrast with the large spines of our regular sea-urchins, 

 the disk-urchins are clothed everywhere with minute, slender, 

 brownish or purplish spines, those on the upper surface being cov- 

 ered with cilia, which cause currents of water to pass continually 

 over the respiratory tube-feet. 



The tube-feet in the clypeastroids are specialized either for 

 respiration or locomotion. Those serving the latter function are 

 scattered all over the upper surface in the interambulacral areas 

 as well as in the ordinary position in the ambulacral areas. On 

 the ventral surface they occur along definite grooves in the am- 

 bulacral areas, with branches extending into the interambulacral 

 areas. 



