194 



MANUAL OF ZOOLOGY. 



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 



3. Morphology of Echinoidea. A, Young specimen of Strongylocentrotus 



'.iensis, viewed from above. B, Small portion of the test of the same, magni- 

 fied. C, Summit of the test of Echinus sph&ra, magnified. D, Clypeaster subde- 

 pressus, viewed from above, showing the petaloid ambulacra. E, Spine of Poroci- 

 daris purpurata. F, Pedicellaria of Toxopneustes lividus. ct a Ambulacral areas ; 

 i i Inter-ambulacral areas ; g Genital plate ; o Ocular plate ; m Madreporiform 

 tubercle ; p Membrane surrounding the anus. (Figs. A, B, and D are after A. 

 Agassiz.) 



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 " circumscript " (ambulacra circumscriptd] or "petaloid" 

 (fig. 93, D). 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 contact with one another. The plates 

 of the test are studded with large tubercles, which are more 

 numerous on the inter- ambulacral areas than on the ambu- 

 lacral (fig. 93, B). These tubercles carry spines (fig. 93, E, 

 and fig. 94) used defensively and in locomotion, which are 

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



