BIHANG TILL K. PV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 19 



of a river, into tlie buccal membrane.' At tlie same time its 

 form will alter. The arehed processes on the inside will spee- 

 <lily 1)6 reabsorbed, its verruca diminish and disappear; but 

 it will expand strongly in breadth laterally, and in depth, and 

 soon be transformed into a thin scale overlapping the prece- 

 ding one, while its verruca will be replaced by marginal ver- 

 tucuIk", and its pedifellar pore sunk into a small and gradn- 

 ally deepening pit. It would be the plaee here to show hovv 

 this process goes on from the first, beginning with the remo- 

 (ielling of the primordial plates into scales, and how it is con- 

 tinued by the descent and transformation of the seconds and 

 so on, but for this specimens are wanting. The nearest ap- 

 ])roach is offered by a Cidaris papillata Leske, already far 

 advanced, having a diameter of 27 mm. It presents in each 

 ambulacrum, from the mouth to the calyx, on an average, fifty- 

 three pairs of plates, and of these the nine first have become 

 free and movable, in the buccal membrane, and transformed 

 into imbricated scales. Another specimen, of 41 mm., has 69 

 pairs of plates, of which 18 detached; a third, of 55 mm., has 

 81 pairs, 15 detached; a fourth, of 62 mm., has 91 pairs, 16 

 detached, thus: 



Diam. "i? mm. 5.'» i)airs. 9 detached 



» 41 -. 69 » 13 » =4 for 16 added = l: 4 



» 55 81 >' 15 > =2 » 12 » =1: 6 



62 91 » 16 » =1 , 10 > rz^l: 10 



from which it appears that the suecessive dismemberment of 

 ambulacral plates is performed at a slower råte in the older 

 specimens, and inversely as the råte of addition of new plates 

 at the dorsal termination of the columns. But all the time 

 the buccal membrane, according as it widens, is filled from 

 the fixed ambulacra with the plates next in age, transformed 

 and overlapping and always bearing pores and pedicels. The 

 five pairs arround the lips are the primordial ones and still 

 recognisable as such. 



In the interradia the new plates are added at the tops 

 of the columns not simultaneously and equally all around the 

 calyx, but in such a manner that in every interradium one of 

 the two columns precedes the other, and has its last comple- 

 ted plate larger or its new-forming additional one more de- 

 veloped. 



' Etndes, p. 28. [>\ XX. lig. 166. 167, 168. 



