STAR-FISH. 



109 



of the Sea-urchin, but instead of being drawn together, and united 

 at the summit of the animal, so as to form a spherical outline, 

 they are spread out on one level in the shape of a star. This 

 change in the general arrangement brings the eye-specks to the 

 extremities of the arms, and places the ovarian- openings in the 

 angles between the arms. The madreporic body is situated on 

 the upper surface of the disk (Fig. 142), at the angle between 

 two of the arms, and consequently between two of the ambulacra, 

 and opposite the odd one. The tube into which it opens, runs 

 vertically from the upper floor of the disk to the lower, where it 

 connects with the circular tube around the mouth, and thus com- 

 municates with all the ambulacral rows. The ambulacral zones 

 which, in the Star-fish, have the shape of a furrow, run along the 

 lower side of each ray (Fig. 141) ; the interambulacral zones 

 are divided, their plates being arranged in rows along either side 

 of the ambulacral furrows. The ambulacral furrow, like the 

 ambulacral zone in the Sea-urchin, is pierced with numerous 

 holes, alternating with each other in a kind of zigzag arrange- 

 ment, one hole a little in advance, the next a little farther back, 

 and so on, and through these holes pass the tenta- 

 cles, terminating in suckers, as in the Sea-urchins, 

 and serving as in them for locomotive organs. The 

 most prominent and strongest spines are arranged 

 upon the large interambulacral plates on both sides 

 of the ambulacral furrows ; but the upper surface of 

 the animal is also completely studded with smaller 

 spines, scattered at various distances, apparently 

 without any regular arrangement. (Fig. 142.) 



The position of the pedicellariae is quite dif- 

 ferent from that which they occupy in the Sea- 

 urchin, where they are scattered singly between 

 the spines and tentacles, though more 'regularly 

 and closely grouped along the lines upon which the refuse food 

 is moved off. In the Star-fish, on the contrary, these singular 

 organs seem to be grouped for some special purpose around the 

 spines, on the upper surface of the body. Every such spine 

 swells near its point of attachment, thus forming a spreading base 



Fig. 141. Star-fish ray, seen from mouth side. (Ayassiz.) 



