ECHINOIDEA. 



227 



though other less important elements are present as well. 

 The " corona " is the main element of the test, and includes 

 all the calcareous covering of the animal except the scat- 

 tered plates round the mouth and anus and the "apical disc." 

 The test is composed of numerous calcareous plates, firmly 

 united to one another by their edges, arranged in rows (fig. 

 117, A), and bearing different names, according to their posi- 

 tion and function. In the curious Urchins which form the 

 family of the Echinotlmridce, and in some of the Palaeozoic 

 Echinoids, the plates of the test overlap one another in an 

 imbricating manner, so that the shell becomes flexible. As 

 a rule, however, the corona forms an immovable case or box, 

 within which the animal is contained ; and its growth is 

 carried on by means of additions made to the edge of each 

 individual plate, by means of an organised membrane which 



Fig. 118. Morphology of Echinoidea. A, Portion of the test of Galerites hemisphericus, en- 

 larged, showing an inter-ambulacral area (a), and an ambulacral area (ft). B, Genital and 

 ocular disc of Hemicidaris intermedia, enlarged : c, Ocular plate; d, Genital plate; e, Anal 

 aperture ; /, Madreporiform tubercle. (After Forbes.) 



passes between the sutures, or the lines where the plates 

 come in contact with one another. 



In all recent and most fossil Echinoids, the test is com- 

 posed of twenty meridional rows of calcareous plates, which 

 are arranged in ten alternating zones or areas (fig. 117, A).' 

 Each zone, therefore, is composed of two rows of plates. In 

 five of these zones (figs. 117, B, and 118) the plates are of 

 large size, and are not perforated by any apertures. These 



