440 THE DEPTHS OF THE SEA. [chap. ix. 



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 

 prolongation downwards of the plated skin which 

 covers the upper surface or ' disk ' of the body. 

 Seated upon the bevelled sides of each radial-axil- 

 lary joint, there is a series of live joints, the last 

 of the five bevelled again like the radial axillaries 

 for the insertion 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 Mliller, 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 accessorv and 



