VIIOKMOSOAIA HISl'IDUM. 



89 



ambitus in this specimen (PI. 48, fig. ,1) as compared with the younger speci- 

 men (PI. 48, fig. ..'). There are fourteen and fifteen ambulacral plates 

 between the buccal plates and the ambitus, and twenty-seven and twenty- 

 eight between the ambitus and the ocular plates. It is only at the abactinal 

 and actinal parts of the ambulacrum that we can still trace the embry- 

 onic arrangement of the plates. From the ambitus towards t lie abactinal 

 pole, with the great lateral extension of the larger ambulacral plate, the 

 second plate has been excluded from the outer edge 

 and the third component plate alone reaches the edge. 

 Four of the abactinal plates are in lineal succession of 

 equal size, and are then followed by three larger plates, 

 each with its two smaller poriferous plates reaching 

 the outer edge, where they are followed by plates 

 having the arrangement characteristic of the older 

 specimens. 



Below the ambitus (PI. 48, fig. 3) on the actinal 

 side, in two of the plates nearest the ambitus are 

 the third plates excluded from the outer edge ; be- 

 tween these and the fifth plate from the actinal sys- 

 tem, the second plate being pushed back of the third 

 plate, the second and third component plates reach 

 the outer edge ; the last five actinal- plates, being 

 nearly equal in size, all reach the outer edge as well 

 as the median line, and alone retain the embryonic 

 arrangement of the younger stage. 



In a specimen of 75 mm. the thirteenth plate in the right zone of the 

 odd anterior ambulacrum is enclosed (PI. 48, fig. 5), while in specimens of 

 120 and 2U3 mm. it is the sixteenth from the actinostome. Fig. 134. 



It will be noticed how interesting is this structure of the ambulacral sys- 

 tem of the Echinothuriae. Nearly all the plates of the actinal part of the ambu- 

 lacrum below the ambitus retain the Ciiftirid arrangement of equal ambula- 

 cral plates one above the other ; while from the ambitus toward the abactinal 

 system these plates form combinations of three, each with a pair of pores, 

 one of the plates being much larger than the others; thus imitating the 

 composite plate of Strongylocentrotus with its pairs of pores, but formed in 

 a very different way, the component plates of Strongylocentrotus being cut 

 through the original plate, while in the Echinothuriaj each of the component 



120 mm. 



Fig. 134. Ph. hispidum. 



