92 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



mosaic. 16 Five double rows of plates, passing from pole 

 to pole, like the ribs of a melon, alternate with five other 

 double rows. In one set, called the ambulacra, the plates 

 are perforated for the protrusion of tubular feet, or 

 suckers, as in the starfish. So that altogether there are 

 twenty series of plates ten ambulacral, and ten inter- 

 ambulacral. The shell is not cast, but grows by the 

 enlargement of each individual plate, and the addition 

 of new ones around the mouth and the opposite pole. 

 Echini live near the shore, in rocky holes or under sea- 

 weed. They are less active than the starfishes ; and feed 

 almost entirely upon seaweed. They reproduce by eggs. 

 Regular Echini,-as the common Arbacia or purple sea 

 urchin, and the green sea urchin, are nearly globular, 

 and the oral and anal openings are opposite. Irregular 

 Echini, as the Clypeaster, are flat, and the anal orifice is 

 near the margin, as in the "sand dollar" or "cake 

 urchin " (Echinarachnius). 



CLASS 4. Holothuroidea 



These wormlike " sea slugs," as they are called, have 

 a soft, elongated body, with a tough, contractile skin 



FIG. 49. -Sea Slugs {Holothitria). 



containing small calcareous plates. One end is abruptly 

 terminated, and has a simple aperture for a mouth, en- 



