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MANUAL OF ZOOLOGY. 



united together, that the entire test becomes flexible and soft. 

 In the typical 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 per- 

 forated by any apertures (fig. 55) ; 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 " pori- 

 ferous zones." Each of these zones is composed of two rows 



Fig- 55- Morphology of Echinoidea. i. Portion of the test of Galerites hemispheric-its 

 enlarged, showing an iriter-ambulacral area (), and an ambulacral area (). 2. 

 Galerites hemispheric-its viewed from above, a Inter-ambulacra ; b Ambulacra. 3. 

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

 ^ Genital plate ; e Anal aperture ; yMadreporiform tubercle.- 4. Spine of the same. 

 (After Forbes.) The tubercles are mostly omitted on figs. 2 and 3 for the sake of 

 clearness. * 



of small plates, which are perforated by minute apertures for 

 the emission of the " ambulacral tubes," or " tube-feet." In 

 one great group of the Echinoids, the ambulacral areas pass 

 from the centre of the base of the shell to its summit, when 

 they are said to be " perfect" (ambulacra perfecta) or " simple." 

 In another great group the ambulacral areas are not thus con- 

 tinuous from pole to pole, but simply form a kind of rosette 

 upon the upper surface of the shell. In these cases as in the 

 common Heart-urchins the ambulacral zones are said to 

 be " circum script " (ambulacra circumscripta] or " petaloid." 

 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 between the sutures, where the plates come into 



