THE SKELETON OF ECHINODEBMS, 101 



showing the skeleton of one of these lithophytes. The natu- 

 ral size of the polypary is seen at fig. 69, and a magnified view 

 of one of the cells, with its rays, is given in fig. 70.] 



In the echinoderms, the test is brittle, and intimately united 

 with the soft parts. It is composed of numerous little plates, 

 sometimes consolidated and immoveable, as in the sea-urchins, 

 or combined, so as to allow of various motions, as in the star- 

 fishes (fig. 36), and in the sea-lilies (figs. 72 and 73), which 

 use their arms both for crawling and swimming. 



Fig. 71. The test of an Echinus. On the right side are seen the 

 spiues and tubular suckers : on the left side, those parts have been re- 

 moved, to show the surface of the test, composed of the ambulacral areas, 

 with the small plates, and poriferous avenues at their margins, and the 

 interambulacral areae, composed of the large polygonal plates. The plates 

 of both areas being covered with tubercles, for supporting spines. 



[In the ECHOTD.E, or sea urchins, the test is of a spherical 

 or pentagonal form, constructed of many series of calcareous 

 polygonal plates articulated together, and divided into two 

 groups, of which five form the ambulacral arese, and five the 

 interambulacral areae, each area being composed of two columns 

 of plates (fig. 71 and 174, d, e). The ambulacral alternate 

 with the interambulacral arese, and they are separated from 

 each other by ten rows of small perforated plates, through the 

 holes of which numerous tubular retractile suckers pass : the 



