440 THE DEPTHS OF THE SEA. [cHAP. 1x. 
inter-radial plates and a row of basals, a large body- 
cavity. The two upper joints of each ray are 
separated from those of the ray next it by a pro- 
longation downwards of the plated skin which covers 
the upper surface or ‘disk’ of the body. Seated 
upon the bevelled sides of each radial-axillary joint, 
there is a series of five joints, the last of the five 
bevelled again lke the radial axillaries for the inser- 
tion of two joints. ‘These five joints form the first 
series of ‘ brachials,’ and from the base of this series 
the arms become free. 
The first of the brachial joints, that is to say, 
the joint immediately above the radial axillary, is, 
as it were, split in two by a peculiar kind of joint, 
called, by Miiller, a ‘ syzygy.’ All the ordinary joints 
of the arms are provided with muscles producing 
various motions, and binding the joints firmly 
together. The syzygies are not so provided, and 
the arms are consequently easily snapped across 
where these occur. This is a beautiful provision for 
the safety of an animal which has so wide and 
complicated a crown of appendages. If one of the 
arms get entangled, or fall into the jaws or claws of 
an enemy, by a jerk the star-fish can at once get rid 
of the embarrassed arm; and as all this group have 
a wonderful power of reproducing lost parts, the arm 
is soon restored. 
When the animal is dying, it generally breaks off 
its arms at these syzygies; so that almost all the 
specimens which have been brought to Europe have 
arrived with the arms separate from the body. 
About six arm-joints or so above the first on 
either branch there is a second brachial accessory and 
