REPORT ON THE ECHINOIDEA. 



57 



following species I have examined with reference to this very point, and find the follow- 

 ing results : — 



The splitting up of the four anal plates into thirteen columnar plates, as is the case in a 

 large Arhaeia nigra from Talcahuano, seems due in that case to the increase in growtli of 

 the anal plates in a confined anal area. All the specimens which have more than four 

 anal plates are fully grown, and I have not found among the many young of Arhaeia 

 punctulata I have examined, while working on the chapter on young Echinids for the 

 Revision of the Echini, a single small specimen with less or more than four anal plates. 

 The largest of these young specimens measuring not quite 5 mm. in diameter down to 

 about 1-.5 mm. when the young had just resorbed the Pluteus. As I have shown in the 

 Revision of the Echini (p. 734, fig. 68), the young Arhaeia has already four anal plates 

 in the earliest stages thus far observed. 



It will be interesting hereafter to observe the growth of the anal plates of such 



(ZOOL. CIIALL. EXP. — PART IX. 1881.) 



18 



