104 



ANNULOIDA. 



to one another by their edges, arranged in rows (fig. 57), and 

 bearing different names according to their position and func- 

 tion. In one or two exceptional cases the plates are so thin 



Fig. 57. Morphology of Echinoidea. i. Portion of the test of Galerites kemisfihericus 

 enlarged, showing an inter-ambulacra! area (a), and an ambulacra! area (). 2. Galentes 

 liemisphericus viewed from above, a Inter-ambulacra ; b Ambulacra. 3. Genital and 

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

 Anal aperture ; f Madreporiform tubercle. 4. Spine of the same. (After Forbes.) The 

 tubercles are mostly omitted on figs 2 and 3 for the sake of clearness. 



and are so united together that the entire test becomes flexible 

 and soft. 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 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 composed 

 of twenty rows of calcareous plates, which are arranged in ten 

 alternating zones or areas (fig. 58). Each zone, therefore, is 

 composed of two rows of plates. In five of these zones (fig. 

 57, i, and fig. 58) the plates are of large size, and are not 

 perforated by any apertures. These zones are called the " in- 

 ter-ambulacral areas." The remaining five zones alternate 

 with the former, and are composed of very much smaller 

 plates, which are perforated by minute apertures or pores. 

 Through these apertures are emitted the little suctorial tubes 

 of the water-vascular system the so-called " ambulacral tubes" 

 or " tube-feet" by means of which the animal moves. Hence 

 these zones of perforated plates are termed the " ambulacral 

 areas" or "poriferous zones." 



