116 MARINE ANIMALS OF MASSACHUSETTS BAY. 



They differ from the Star-fishes in having the disk entirely 

 distinct from the arms ; that is, the arms, instead of merging 

 gradually into the disk, start at once from its margin. They 

 have no interambulacral spaces or plates ; but the whole upper 

 surface is formed of large hard plates, which extend from the 

 back over the sides of the arms to their lower surface, where 

 they form a straight ridge along the centre. (Fig. 149.) The sides 

 of these plates are pierced with holes, through which the ten 

 tacles pass ; these have not, like those of the Star-fishes and 

 Sea-urchins, a sucker at the extremity, biit are covered with little 

 warts or tubercles (Fig. 150) ; they are their locomotive ap- 



Fig. 150. 



pendages, arid their way of moving is curious ; they first extend 

 one of the arms in the direction in which they mean to move, 

 then, bring forward two others to meet them, three arms being 

 thus usually in advance, and then they drag the rest of the body 

 on. They move with much more rapidity, and seem more active, 

 than the Star-fishes ; probably owing to the greater independence 

 of the arms from the disk. The spines project along the mar 

 gin of the arms, and not over the whole surface, the back of the 

 arms being perfectly free from any appendages, and presenting 

 only the surface of the plates. The madreporic body is formed 

 by a plate on the lower side of the disk, in a position correspond- 



Fig 149. One arm of Fig. 148 ; from the mouth side. 

 Fig. 150. Ambulacral tentacle of Ophiopholis ; magniaed. 



