PROFESSOR WYVILLE THOMSON ON T1IE ECHINODERMATA. 407 
and its scattered oil-cells and endoplasts more numerous and distinct. 
The inner organized granular layer increases in thickness till only a 
small central space is filled with granular fluid, while between it and 
the external layer, or in the external layer itself, small plates of 
characteristic calcified areolar tissue are irregularly scattered. 
The star-like form now becomes still more distinct, a regular 
series of calcareous plates are developed on the dorsal surface, one 
large and rapidly expanding plate at the end of each ray, and a 
smaller one at each of the re-entering angles. On the lower surface, 
a pair of plates, each with a concave edge towards the point of the 
ray, and a convex one towards the centre of the star, are formed at 
the base of each arm, so that the two plates of a pair unite in the 
centre of the ray, while their free ends meet the free ends of the 
adjacent plates of the next pairs, forming a calcareous inter-radial 
angle, projecting into the central space. These plates are rapidly 
followed by a double row of almost linear plates with double concave 
edges ; which extends towards the point of the ray, leaving between 
every two pairs, two opposite apertures for the passage of the pedal 
vesicles. 
While these plates are being developed, a tubercle appears on the 
oral surface at the base of each arm, and a delicate circular vessel 
forms a slightly raised ring round the centre. This ring, in one part 
of its course, passes under or blends with, and is lost in the base of 
the peduncle. 
The tubercle at the base of the ray now takes a crescentic form ; 
and shortly the crescent resolves itself into three tubercles, two oppo¬ 
site and occupying either side of the median line of the ray, the other 
in the centre of the ray and connected with the circular ring by a 
delicate straight tube. This central tubercle next becomes slightly 
crescentic, and resolves itself into three tubercles, which arrange 
themselves like the first three, and in this way a central vessel, pro¬ 
ceeding from the ring, follows the development of each ray, with a 
row of tubercles on either side. These tubercles are shortly deve¬ 
loped into suckers like those of the tubular feet of the peduncle, 
and supported by precisely similar transparent contractile tubes, filled 
with the same fluid, in which chyle-globules revolve and circulate in 
exactly the same way. During these changes the peduncle remains 
unaltered. The embryo stands upon its three feet like a miniature 
three-clawed drawing-room table. 
26. Circulation of granules takes place rapidly in the peduncle 
and appendages, but pressure applied to the starfish will no longer 
send the granular contents of the disc into the peduncle, while pres¬ 
sing the peduncle does not inject the general cavity of the star, but 
only renders turgid the circular canal and the radial ambulacral 
vessels. A change now begins to take place in the peduncle. It 
becomes more flaccid, and frequently portions of the tubular feet are 
separated by deepening constrictions, and shortly all that remains is 
an inflated sac hanging to the under surface of the disc. The further 
