56 C. F. Liitken on Ophiurans. 
the joint, just before the fork, has two discs, instead of one; 
on either side; but the halve rmost joint of al are 
ee sundered, and, inelinin to bith right and left, are 
so 1 to the correspon fing pieces of the cee: eb arms on 
oe ioe ( penile sily ” Mille, in the “System der 
Aste 
uses the ler” at random for mouth- 
frames and jaws. These parts are pgp 4 visible on the out- 
side, but, in Ophioderma and allied genera, they are covered 
with grains. All the rest of the interior aksletes is hidden by 
the skin-skeleton. Miiller and Troschel, in the same work, point 
out the homology between the discs in the arms of the Ophiuree 
and the joints in those of star-fishes; but as they started with 
the idea that these joints constituted ‘a true internal cope 
they came to the opinion that this was peculiar, and no 
found in any other Kchinodermata. Gaudry, also, does a con- 
sider the interior skeleton of Ophiurans as homologous with oe 
bulacral plates, but looks on it as a special structure in serpent- 
stars. It is in the side arm-plates that he finds the homologues 
of the ambulacra. 
The skin-skeleton proper is to be found in the scales on the 
disc, spre genital plates and the four rows of plates on the arms 
called upper, ander and side plates (seutella dorsalia, ventralia, 
corres To the jointed structure of the interior arm-skeleton 
corresponds, Sonsebnontly, a similar one in the skin-skeleton. 
per, an ae and two side plates together form a joint, 
an this eo ds to a joint of the interior skeleton, except 
5 eee, beyond their proper joint to the next outer 
jot a Ses ne lates sometimes lie side by side, but again the 
Iternate Se the others, particularly when the 
foot a As to their form, the upper p 
