350 
POPULAR SCIENCE REVIEW. 
side arm plates do not encroach, over- much, upon the upper and 
lower surfaces of the arms, the internal skeleton is well deve- 
loped ; but where the side arm plates form a large portion of 
the arm, then the elements of the structure inside are feebly 
represented. They are not wanted, for a dense squamose 
covering capable of but slight movement exists. But when the 
ordinary mobility of the arm from side to side and a little 
of an extraordinary kind from the horizontal downwards are 
possible, and the outside plates are not too close or rigid, then 
the long line of ossicles inside each arm is perfect. The resem- 
blance of the inner skeleton of the arm to the vertebrae of the 
higher animals is, of course, only superficial, but nevertheless 
some of the methods by which vertebrae are joined together in 
the Ophidia and mammalia are foreshadowed in it. On breaking 
the arm of an Ophiuran across, near the body where one joint 
is connected to another, that is to say, between two consecutive 
lower or upper arm plates, the central part is seen to be filled up 
with a rather complicated structure or ossicle, which presents two 
surfaces. One, seen on the broken face of the arm left attached 
to the body, is the external, and another, on the separated piece, is 
the internal, and they fit one on to the other and are joined 
during life by muscles. The contour of the ossicle of each joint, 
conforms to the outline of the arm, in transverse section, and its 
length corresponds to that of the outer plates. The ossicle 
adheres by the agency of fibrous tissue and muscle to the 
upper arm plate above, and to the other plates in their proper 
positions : but it has a decided groove on its under surface in 
the median line, and this is turned into a canal by the contact 
with the upper surface of the lower arm plate. A smaller 
groove is noticed above, and is beneath the lower surface of 
the upper arm plate. These two grooves are in a vertical line 
which would separate the ossicle into halves, and the lower one 
contains the extension of the water system and nerve from 
within the body. The surface of each ossicle has certain knobs 
and pegs, cavities for their reception, and broad spaces for 
muscular attachments. Thus the inner surface has rather above 
the middle, yet centrally, a broad projecting knob, and near the 
lower edge two smaller knobs, one on each side of the median 
line, in which and between them there is a depression or socket. 
On the external 6ide of the ossicle three cavities exist, which 
correspond with the knobs, and they fit them, and there is also 
a peg which fits into the socket in the other ossicle. Rotation 
or twisting of the arm is thus prevented, and also too much 
up and down movement, whilst some latitude exists for side to 
side motion. Considerable surfaces exist on both faces, into 
which muscles are fixed, and there are muscular fibres on the 
outside of the ossicle attaching it to its fellows before and behind. 
