TllE SKELETON OF ECHDSTODEEMS. 



101 



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

 ral size of the polypary is seen at fig. 69, and a magnified vie-* 

 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 

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

 moved, to show the surface of the test, composed of the ambulaeral ares, 

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

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

 of both area? being covered with tubercles, for supporting spines. 



[In the ECHrNiDj;, 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 ambulaeral areae, and five the 

 interambulacral arese, each area being composed of two columns 

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



