ECHINODERMATA. 
757 
not oxtonsilo, slightly club-almpcd towards the distal extremity, which is 
fringed on either side by a single row of short conical tubercles. The base 
of these tentacles is involved in the contractile sarcode-ring surrounding the 
mouth. When the disk is fully expanded they lie in pairs up against the 
inner surface of the oral lobes. They are frequently, however, gathered in- 
wards together, or singly curving over the mouth. They form part of a very 
characteristic system of “ non-extensile tentacles,” which afterwards fringe 
the radial and brachial grooves. At this stage, then, the oral ring usually 
gives off twenty-five tentacular appendages, of which fifteen are radial and 
extensile, and ten are interradial and non-extensile. 
Imbedded in the sarcode at the base of each of the azygous tentacles, a 
peculiar glandular body is very early developed. At first it consists of a 
minute vesicle containing a transparent fluid. The vesicle gradually increases 
in size till it attains a diaineter of about 0 08 millim. in diameter. Its con- 
tents become granular ; and at length it has the appearance of a large cell 
with a special wall, included in a capsule formed of a fine sarcode-layer, from 
which the cell can he turned out unbroken. 
The stem now gradually lengthens, by additions to either end of the sheaf- 
like calcareous cylinders which form the axis of the stem-joints, and by the 
addition of new rings which rapidly become filled up by the vertical tissue 
at the top of the stem, immediately betieath the rudiment of the centro- 
dorsal plate. The disk of attachment becomes opake by the addition of cal- 
careous matter, and is firmly fixed. The centrodorsal ring is more definite 
in form, though it is still simply perforated in the centre, and in connexion 
with the sarcode-axis of the stem, and bears no traces of dorsal cirri. The 
basals expand and form a wide, nearly continuous cup. By the rapid ex- 
pansion of the body, five diamond-shaped spaces are left at the points where 
the upturned angles of two oral plates are opposed to the bevelled-off upper 
angles of two adjacent basals. In these spaces C 3 dindrical spicula appear, 
which soon become club-shaped, dichotomize, branch, and anastomose into 
delicate net-like superficial plates, irregularly oval, slightl}" produced supe- 
riorly'^, their upper, narrower portions resting beneath, and supporting the 
gi’adually extending sarcode -projections which are terminated by^ the azygous 
tentacles. The equatorial portion of the body, the band between the upper 
edges of the basals and the lower edges of the orals, now rapidly expands. 
The five young arms extend outwards, their bases carrying out with them a 
zone of sarcode which gives the central portion of the body^ a great additional 
width. The oral plates maintain their original position ; so that they are now 
completely separated from the basals by this intervening equatorial band, 
and are left (a circle of five separate plates, each enclosed in its sarcode-lobe) 
on the centre of the upper surface, surrounding the mouth and enclosing the 
ten non-extensile tentacles only^ The first radial plates begin to thicken, 
especially' towards the upper margin ; and this thickening is produced by^ tlm 
growth, beneath the cribriform superficial calcareous film, of a longitudinal 
mass of tissue of the same character as that which forms the cylindrical axis 
of the stem-joints. On the lower surface of each arm, in linear series, imme- 
diately above the first radials, two spicula, horseshoe-sliaped, with the open- 
ing above, appear almost simultaneousiy'^, and become quickly filled up with 
elongating sheaves of longitudinal trelliswork. These extend along beneath 
the extending arm.s, and indicate the second radials and the radial axillaries. 
