BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 29 



plate. From this lobe it will separate and in its turn descend 

 alone into the buccal membrane. Thus, at this stage, all the 

 ten parting plates — their lobes apparently being left and 

 reabsorbed, — are demi-plates. As sucli tliey begin to form 

 into scales. Of these the five buccal columns I a, II «, III 

 b, IV a, V b, Pl. XII, fig. 161, contain each seven fuUy de- 

 veloped scales, of which the last, the seventh, is a transformed 

 demi-plate 3, and this is followed by the detached poral part 

 of one of the five entire plates in the act of parting. The 

 columns I b, II b, III a, IV b, V a, on the other lianu, con- 

 tain each eight scales, of which the last-formed, the eighth, 

 derived from a transformed demi-plate 2, is unequally deve- 

 loped, not far from fiilly so in III a, V a, II b, less so in I 

 b and IV b. They all soon conform to the new order and 

 become regular constituents of the flexible bnccal membrane, 

 taking the shape of adorally overlapping scales. Every one 

 of these is of an angular form, the inner branch being the 

 shorter. Their aboral, narrow, raised margin bears a row of 

 verruculcie, while the adoral submerged part is produced into 

 a pointed lamina, fi(/. 161, projecting aborall}' beneath the 

 following scale. The five triangulär sets, each consisting of 

 two ambulacral columns, occupy the whole of the buccal mem- 

 brane and are contiguous to one another, their boundary lines 

 coinciding with the mesial sutures of the interradia. 



It is readily seen that in all this the heterotropy is strictly 

 maintained. It pervades the wliole of the ambulacrum. 



The dorsal pores, Fl. XII, fig. 165, 166, have the peri- 

 podium only little marked. The perforations are nearly equal, 

 approximated, the inner one separated adorally by a very thin 

 parti tion from the meatus of the ambulacral nerve; the bridge 

 is narrow. Dorsally and at the ambitus their position is trans- 

 verse, ventrally and towards the peristome more and more 

 diagonal, fir/. 164. When being detached into the buccal 

 membrane they become almost longitudinal, fig. 162, 163, and 

 unequal, the aboral one diminishing, while at the same time 

 the peripodium becomes more marked, contracted, rounded, 

 and deeper. 



Thus the ambulacral system of the Asthenosoma, taken 

 as a whole, is thoroughly different from that of any other of 

 the Ectobranohiates, in the constitution of its compound plates 

 and the arched rows of the pores, as in the existence of de- 



