NOTES ON THE OPHIURANS, OR THE SAND AND BRITTLE STARS. 351 
On looking at the top of an ossicle, underneath the upper arm 
plate, the small groove is seen, and on either side of it a 
broadish angular flap on which the upper arm plate rests and 
which is the homologue of the mouth or jaw frame. Inter- 
nally there is a concavity for the reception of the peg of the 
next ossicle within, and externally is seen the peg itself. The 
lower surface of an ossicle shows the large median groove for 
the water tube and nerve, and there are two marks, one on 
either side, where the tentacles rest as they pass outwards to 
the surface, and also the peg and socket in their relative positions 
fore and aft. 
There is an offshoot of the water tube and nerve to each 
tentacle, and there are nervous filaments crossing the muscles 
which are on either side of the median pegs and holes. The 
decalcification of an arm in a small specimen will render the 
vessel and nerve visible, and it appears that they are enclosed 
in rather thin fibrous tissue, the nerves being numerous, exces- 
sively delicate, with numerous nuclei, and they surround the 
water tube. (Fig. 8 x, y> z.) This is cylindrical, but it swells 
at the joints of the arm plates where the branches are given 
off, and its walls are thick, transparent, and sparely nucleated. 
(Fig. 80.) The muscular fibres are unstriped, and most are 
simple ; hut many are compound and branch and unite, forming- 
a plexus. Many extremely delicate and narrow fibrils wander 
about the muscular fibres, and probably they are nerves. More- 
over the impression remains on the eye, that there is an open- 
ing at the joint between the meshes of the muscles and the water 
tube, as if the fluid could pass into the tissue. 
In the instances where the plates of the arms are not well 
developed, as in the genus Ophiothela , whose species cling to 
Isidinse and even to other Ophiurans, and in which skin and 
grains of carbonate of lime almost replace the plates, the 
ossicles are still developed. The arms are of great importance, 
and they form, by a morphological metamorphosis, the jaws 
and teeth and support the nervous and vascular systems. A 
few words must suffice to notice the spines of the arms. 
They are situated on the side arm plates, and on the front, free 
outer margin : they are close to the side and point along the 
arm in Sand Stars, which have but slight lateral movement in 
the arms ; and they project more or less at right angles from 
the arm in the other Ophiurans. Those bristly-looking arms, 
characteristic of Brittle Stars, are competent to move and 
wriggle sideways to any extent. Exquisitely beautiful are some 
of these spines, and especially when they are of a glassy trans- 
parency. They frequently move on a rough ball and socket- 
joint, and are often numerous and even crowded, or are placed in 
one row. Besides these accessories to the side arm plates, there 
