ECHINOIDEA 255 



divided into 10 regular areas, each formed by a double series of 

 plates. Five of these areas carry each two series of tube-feet, and 

 thus correspond to the arms of sea-lilies, sea-stars, and brittle- 

 stars. These areas accordingly are radial and are designated the 

 amhulacral areas or simply ambulacra ; the areas situated alter- 

 nating between the ambulacra are interradial and are designated 

 the interambulacral areas or interambulacra. In each area there 

 is usually a double series of larger " primary " tubercles. Generally 

 the interambulacra are conspicuously broader than the ambulacra, 

 and their tubercles distinct^ larger than those of the latter. In 



Fig. 139. — Test of Echinus elegans, from the under or oral (1) and the upper 

 side (2). The areas with the two series of small tubercles are the 

 ambulacra, those with the two series of larger tubercles the inter- 

 ambulacra. Outside the tubercles in the ambulacra are seen the pores 

 of the tube -feet. In the middle of the upper side is seen the apical 

 system, in the middle of under side the mouth area, in the centre the 

 mouth with the five teeth. Round the mouth opening are the five pairs 

 of buccal plates, and outside these very small, scattered plates in the 

 skin of the peristome. In the edge of the mouth area are seen five 

 pairs of small cuts in the test, the gill -cuts. Slightly diminished. 

 (From Danmark's Fauna.) 



the oldest known Echinoid {Bothriocidaris) the interambulacra 

 have only a single series of plates, and a vestige of this arrange- 

 ment is still found in the other Echinoids, in the first inter- 

 ambulacral plate always being single (in some forms it disappears 

 during growth). In the other palaeozoic forms there are more 

 than two series of plates, either in the ambulacra or in the 

 interambulacra ; all other Echinoids, with a very few exceptions, 

 have two series of alternating plates in each area. In several 

 forms the plates of the ambulacra coalesce, forming thus com- 

 pound plates, which carry as many tube-feet (and pore-pairs) as 

 there are small plates {primary plates) contained, each ambulacral 

 plate having originally one tube-foot. In such compound plates 

 the tube-feet are arranged in regular arcs, 3 or more, up to 9-12, 



