92 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BlllL 



disk (Plate XXIII), and in the key-hole urchin it is very near 

 the mouth, on the proximal border of the middle lunule (Plate 

 XXIV, fig. i). In neither of these forms, therefore, is the body 

 radially symmetrical, and they belong to the order Clypeastroidea, 

 or disk-urchins, as they may be called. 



Both these species of clypeastroids have strong jaws, with fine, 

 sharp, calcareous teeth, but the jaws (Plate XXIV) are less com- 

 plicated than in the order Regularia. The peristome is very 

 small, so that the jaws cannot be opened so widely as in the reg- 

 ular forms. By means of the teeth both these species pick up 

 particles of sand and mud, with the diatoms and other minute 

 organisms adhering. These particles are swallowed in vast num- 

 bers, and the organic matter digested out, after which the sand is 

 passed out of the anal opening. 



Both of our species of disk-urchins are plainly bilaterally sym- 

 metrical, the mouth and anus lying in the median antero-posterior 

 plane. This bilaterality is shown also in the outline of the body. 

 As illustrated in Plate XXIV, the anus in Echinarachnius lies in 

 an indentation at the posterior end of the disk, and there are two 

 other slight indentations on the edge of the disk in the adjacent 

 ambulacral areas. In Mellita (Plate XXVIII) the manifestation 

 of bilateral symmetry is carried still farther, for here there are 

 five symmetrically placed lunules, one being in the posterior inter- 

 radius, while the others occupy the two adjacent radii on each 

 side. These arise in the young urchins as notches in the edges 

 of the test, and gradually deepen and close over as the animal 

 increases in size. Furthermore in both species the disk is 

 broader posteriori}'', and narrower in front. 



Although the anal opening has moved from its primitive 

 position at the aboral pole, the ocular and genital plates have 

 retained the same position as in the regular sea-urchins. They 



EXPLANATION OF PLATE XXIII. Skeleton of Disk-urchin, Echinarachnius 



parma. (Natural size.) 



Aboral and oral surfaces with spines removed. The upper figure shows 

 the ambulacral areas ("petals") and the four genital pores near the 

 center of the disk. The lower figure shows the mouth and teeth, and the 

 closely fitted plates. The indentation on the lower median border indicates 

 the position of the anus. 



(Photographs loaned by the U. S. Fish Commission, with permission of 



Dr. H. L. Clark.) 



