338 Descriptive Zoology. 



The arms are not hollow as in the starfish, there is no 

 ambulacral groove and the tube feet project on the side 

 instead of on the oral surface. On account of the taper- 

 ing arms, with their active wriggling movements, the 

 brittle stars are sometimes called serpent stars. They 

 are also called sand stars. In some ophiuroids the arms 

 are branched as in the common basket star, whose arms are 

 many times branched, and become so inrolled as to give 

 the name basket-fish. 



CLASS III. ECHINOIDEA. 



Occurrence. Sea urchins are found along the Atlantic 

 coast from low-water mark to fifty fathoms, being more 

 common among rocks. They are also found clinging to the 

 piles of wharves. The northern form is greenish, with 

 slender spines, somewhat resembling a chestnut bur. The 

 more southern form is of a dark color, with fewer and 

 stouter spines. 



General Form of a Sea Urchin. The common sea urchin 

 is apple-shaped, the mouth being where the stem of an 

 apple is, and the anus at the opposite end, or pole, as the 

 ends are termed. Running from the mouth to the anus are 

 meridians, marked especially by the five double rows of 

 tube feet, which, when fully extended, are long and slender, 

 reaching beyond the tips of the longest spines. For some 

 considerable space around the mouth is a leathery mem- 

 brane, the peristome, where the skeleton is undeveloped. 



The Skeleton. As in the starfish, the leathery body wall 

 abounds in limy plates, or ossicles ; but instead of bein{ 

 loosely attached to each other, as in the starfish, they ai 

 in the sea urchin, firmly cemented together, constituting 

 rigid shell. These shells are sometimes sold under tl 



