ANNULOIDA : ECHINODERMATA. 



145 



the plates of the test are so thin, and are so united together, 

 that the entire test becomes flexible and soft. In all recent 

 members of the order the test is composed of twenty rows of 

 these plates, arranged in ten alternating zones, which pass 

 from the one pole of the animal to the other, each zone being 

 composed of two similar rows. Five of these double rows are 

 composed of large plates, which are not perforated by any 

 apertures (fig. 39) ; the zones formed by these imperforate 

 plates being termed the " inter-ambulacral areas." The other 

 five double rows of plates alternate regularly with the former, 

 and are termed the " ambulacral areas," or " poriferous zones." 



Fig. 39. — Morphology of Echinoidea. i. Portion of the XR&'i.oiGaleriteshtmisphericm 

 enlarged, showing an inter-ambulacral area (a), and an ambulacral area {b). a 

 Galerites hemisphericus viewed from above, a Inter-ambutacra ; b Ambulacra. 3. 

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

 Genital plate ; e Anal aperture ; J Madreporiform tubercle. 4. Spine of the same. 

 (After Forbes.) The tubercles are mostly omitted on figs, z and 3 for the sake of 

 clearness. 



Each of these zones is composed of two rows of small plates, 

 . which are perforated by- minute apertures for the emission of 

 the " ambulacral tubes," or " tube-feet." Growth of the test is 

 carried on by additions made to the edge of each individual 

 plate, by means of an organised membrane which passes be- 

 tween the sutures, where the plates come into contact with one 

 another. The plates of the test are studded with large tuber- 

 cles, which are more numerous on the inter-ambulacral areas 

 than on the ambulacral, and are wanting on all the plates 

 which do not belong to either area. These tubercles carry 

 spines (fig. 40), used defensively and in locomotion, which are 

 articulated to their apices by means of a sort of "universal" 



