152 CIDARITES. 



anus ; each of these has a little triangular depression, and 

 a perforation at its lower part. When the animal is alive 

 these perforations are seen to he filled up by a red mem- 

 brane, or substance ; and I regard them as analogous to 

 the so-called eye of the Starfish. They are placed exactly 

 in the corresponding position, — namely, at the extremities 

 of the avenues of suckers, each pair of avenues meeting in 

 fact at these little plates. There are five pairs of avenues ; 

 they run from mouth to anus. The plates which separate 

 the two avenues of each pair are called ambulacral ; and 

 those which separate the pairs, forming a space of double 

 the width or more, are called inter-ambulacral. Both am- 

 bulacral and inter-ambulacral plates are studded with 

 mamillary spiniferous tubercles. On each ambulacral plate 

 there is usually one large tubercle and three or four small 

 ones ; on each inter-ambulacral plate there are from five 

 to seven large tubercles and several small ones. The spines, 

 which crown these tubercles are all of one form, though 

 of various dimensions as regards thickness. They are all 

 finely striated longitudinally, and have the ridges narrower 

 than the furrows, which latter are finely striated across. 

 There is generally one small tubercle separating the trans- 

 verse rows of pairs, which are oblique, three pairs in each 

 row, the rows running parallel to each other. Small 

 ridges and furrows also separate the pairs of pores from 

 each other. The avenues are broadest towards the middle 

 of the animal, smallest towards the mouth and the apex. 

 Through the pores protrude the suckers, which are long, 

 cylindric, and provided with a circular disk at their extremi- 

 ties. The number of these suckers is very great. In a moder- 

 ate-sized Urchin I reckoned sixty-two rows of pores in each 

 of the ten avenues. Now, as there are three pairs of pores 

 in each row, their number multiplied by six, and again by 



