NOTES ON THE OPHIURANS, OH THE SAND AND BRITTLE STARS. 349 
jaws of two angles, or, in other and more correct words, the jaws 
are modified arm-endings, being composed of two arm joints, 
the inner one of which is bent down so as to be lower than the 
other.* If a side look can be got at a jaw angle (fig. 9), by 
cutting an Ophiaran across the body along the line of one of the 
arms, the jaw will be seen to be deep and to extend very visibly 
from above downwards. On the side, are two holes and two 
scales or scale-like spines, an upper and lower, and they are for 
the upper and lower mouth tentacles. These long, slender, 
fleshy, hollow bodies come out through the holes, and their 
tubular cavities are continuous with the circular water canal, or 
rather with an offshoot which makes its way to the arm, passing 
obliquely downwards and outwards to reach a groove, which, in 
the natural position of the Ophiuran, is just above the lower 
arm plates in the body of the arm and in the median line. As 
each of the five main branches passes off from the circular water 
canal, close to the rim of the stomach membrane, to reach its 
special arm, the side branches for the first two tentacles are 
given off, one after the other. This dipping downwards and 
outwards of the main branches, is accompanied by a similar 
distribution of nerve, and the nerve cords cling to the water 
systems, being enveloped in fibrous tissue in a common sheath. 
Every tentacle in the arm has its offshoot from the radial vessel, 
and it is short and ends in a cul de sac , the tentacle not being 
open at its tip. There is no mechanism to produce a sucker, 
and those organs are wanting; consequently there are no 
extra contractile sacs associated with the tentacles and in con- 
nection with the water system, as is the case in the Star-fish. 
The radial water canal passes, then, along each arm, associated 
with nerves, and at each joint a water tube and nerve are given 
off, on either side, to the tentacles. Probably the tentacles have 
something to do with respiration. The arms, as already noticed, 
consist of a series of four plates, united more or less closely by skin, 
and there is an opening at each joint or cale on either side 
interiorly and between the side arm plates and the lower arm 
plate. The member thus enclosed in armour in many genera, 
and having much visible skin between the plates in others, has an 
internal skeleton, the explanation of whose ancestry defies the 
most imaginative biologist. It is a structure per se in the Echi- 
nodermata, and is as elaborate a piece of mechanism as can well 
be imagined. The use of it is evident enough, and there is 
some correlative relation between its elaboration and that of 
the side arm plates. That is to say, in those genera where the 
* There is a capital diagram of this by Theodore Lyman, in “ Ophiuridre," 
Ac. — Bulletin of the Museum of Comparative Zoology at Harvard College, 
Cambridge, Mass. Yol. iii. No. 10. 
