9l MK. r. ]I. CARPENTER OX THE GENUS ACTINOMETRA. 



iutcrnal faces of the first radials, its central portion is also frequently connected with 

 the radial pentagon by delicate processes, which sometimes sprout forth irregularly 

 from the inner margins of the component pieces of the latter ; but sometimes form a 

 more regular ingrowth, which considerably contracts the central space on the ventral 

 aspect of the disk, and becomes continuous with an annular projection from the ventral 

 face of the rosette. 



(§ Gi) Before attempting to understand the complicated condition of the basals in 

 Actinometra, it will be well to study the mode in which the embryonic basals of Ant. 

 rosacea become metamorphosed into the rosette, as described by Dr. Carpenter, from 

 whose memoir the following account is principally taken. 



In the young animal each basal is a flattened irregularly pentagonal plate, the apex of 

 which lies between a pair of radials that partially rest upon it. On the ventral surfaces 

 of the basal plates lie the five primary basal cords V, W, X, Y, Z, proceeding from the 

 angles of the quinquelocular organ. Each of these divides into two branches, F",, V2, 

 .... ^1, Z.,, the secondary basal cords, which pass on to the ventral faces of each pair of 

 contiguous radial plates, e. rj. X., and Y-^ to one radial, Y2 and X^ to the next, and so on. 



Both basals and radials gradually become much thickened by an endogenous extension 

 of the calcareous nctw^ork, which takes place in sucli a manner that the basal cords 

 come to lie in furrows channelled out on the ventral surfaces of the plates. By a further 

 endogenous growth of the radial plates these furrows are converted into canals (the 

 " central canals " of Johannes Miiller), which at first lie close under the ventral surfaces 

 of the plates, but come gradually, by a continuation of the same process, to lie in their 

 central axis. 



In the basals, however, this process of endogenous growth is followed by one of 

 absorption ; for the criln-iform film of which each basal is originally composed, and 

 which still forms its external layer, now undergoes absorption, especially in its central 

 portion, where it covers in the dorsal side of the primary basal cord ; so that the central 

 space left by the incomplete union of the proximal ends of the five embryonic basal 

 plates is extended on its dorsal aspect into five broad rays, though on its ventral aspect, 

 where it is bounded by the last-formed portion of the endogenous reticulation, it shows 

 no corresponding increase. It is this last-formed ventral portion which persists in the 

 adult as the five triangular interradial processes of the rosette (PI. IV. figs. 3, 7, 13, 

 16, o')- 



The formation of the five radial or spout-like processes is somewhat more complicated. 

 The removal of the external layer in the centre of the dorsal aspect is carried so far as 

 to leave nothing but a kind of thickened margin along those sides of the plate Avhich are 

 received between the first radials ; and by an extension of the same process along the 

 median dorsal line of each plate as far as its salient angle, so as completely to remove 

 the terminal portion of its inferior or dorsal layer, its two lateral portions become sepa- 

 rated from each other at their distal ends, and remain as small curved processes extend- 

 ing outwards. Those of every two contiguous basals now unite to form a sort of ray 

 ciu'ving towards the dorsal aspect ; and this is the rudiment of one of the five radial or 

 spout-likc processes of the rosette, the shape of which becomes much more strongly pro- 



