SEA-URCHIN. 



103 



ing into a complicated intestine to be presently described. From 

 the mouth, the ten zones diverge, curving upward to meet in the 

 dorsal area on the summit of the body. (Fig. 133.) 



Fig. 132. Fig. 133. 



Let us now examine the appearance and functions of the various 

 appendages on the surface. The tentacles have a variety of func- 

 tions to perform ; they are the locomotive appendages, and for 

 this reason, as we have seen, the zones along which they are placed 

 are called the ambulacra, while the intervening spaces, or the 

 broad zones, are called the interambulacra. It should not be sup- 

 posed, however, that the locomotive appendages are the only ones 

 to be found on the ambulacra, for spines occur on the narrow as 

 well as on the broad ones, though the larger and more prominent 

 ones are always placed on the latter. The tentacles are also 

 subservient to circulation, for the water which is taken in at the. 

 madreporic body passes into all the tentacles, sometimes called on 

 that account water-tubes. Beside these offices the tentacles are 

 constantly busy catching any small prey, and conveying it to 

 the mouth, or securing the bits of sea- weed with which, as has 

 been said, these animals conceal themselves from observation. 

 It is curious to see their fine transparent feelers, fastening them- 

 selves by means of the terminal suckers on such a floating piece 

 of sea-weed, drawing it gently down and packing it delicately 

 over the surface of the body. As locomotive appendages, the ten- 

 tacles are chiefly serviceable on the lower or oral side of the ani- 

 mal, which always moves with the mouth downward. About this 

 portion of the body the tentacles are numerous (Fig. 134) and 

 large, and when the animal advances it stretches them in a given 



Fig. 132. Portion of shell of Fig. 131, with spines rubbed off. (Ayassiz.) 

 Fig. 133. Sea-urchiu shell with all the spines removed. (Ayassiz.) 



