204 ARBACIA PUNCTULATA. 



high, the tubercles of the interambulacral space distant, separated by crowded 

 m diaries, rarely forming at ambitus more than three rows on each side of the 

 median line, of which usually but one row reaches the apex ; though in the 

 specimens from the northern localities we frequently have two and even three 

 of the vertical rows encroaching upon the bare areas reaching well towards 

 the abactinal region. The spines are long, moderately stout, frequently 

 exceeding in length the diameter of the test. 



The color, when alive, varies greatly; from a deep violet, almost black, to a 

 straw-color, or spines tipped with violet>brown ; dry, denuded tests are usually 

 grayish, with more or less pinkish tint near apex, or light greenish-gray; the 

 poriferous zones being in each case of darker tint than test. 



Midler was the first to call attention to the difference in the tentacles of 

 the abactinal part of the ambulacra! system of this genus. They are pointed, 

 as in Diadema, having no sucking disk, and when fully expanded (as in 

 PI. V. f. /, .',.;) give to the Sea-urchin a very striking appearance, forming 

 tufts extending far beyond the spines of that pari of the test. These pointed 

 tentacles are flattened in one direction ( /'/. V. f. 6, 7, 8), they are capable of 

 enormous expansion (PI, V. /..;). but do not appear to act as rapidly as those 

 provided with suckers, which contract at least contact, while the pointed 

 tentacles can be considerably disturbed before they are drawn in. 



The mode of moving of Arbacia is quite different from that of our 

 common Strongylocentrotus ; instead of slowly dragging itself along by 

 means of the suckers of the actinal surface, it makes free use of its spines, 

 and by a sort of tilting motion advances quite rapidly. The spat Inform 

 shape of the spines around the actinostome in species of this genus is un- 

 doubtedly due to the wear and tear produced by this means of locomotion,* 

 which enables them to travel quite rapidly over the ground, though the 

 slower mode of propulsion by means of the sucking tentacles of the actino- 

 stome is not unfrequently emploj'ed by them when climbing. 



* There is formed a sort of cap at the extremity of the actinal spines, found in all the species of 

 Arbacia, which Desmoulins lias figured for several species in the Actes de la Soc. Linn, de Bordeaux 

 for 1870, and which is replaced as fast as worn out. 



