278 
S LADEN : STRUCTURE OF ASTEROIDEA. 
ambulacral plates, and these open into the upper bag-like portion 
of the sucker feet, — which latter may be described as simple sacs, 
having very extensile and strongly muscular walls. It will thus 
be seen that the whole system of vessels is filled with fluid ; and 
this is known as the ' water- vascular system.' (Coloured blue in 
the diagram.) "When the starfish wishes to extend its sucker, 
muscular pressure is exerted upon the bag-like portion of the sac ; 
this causes the contained water, and with it the tentacle-foot, to 
be squeezed forwards in consequence of the contraction of the 
reservoir-bag. "When it is necessary to retract the foot, the reverse 
of this process takes place. Returning to the section, — at the outer 
extremity of each ambulacral ossicle is a small plate (Fig. 1, b.), 
and the series of these form the margins of the furrow. They are 
called 1 adambulacral ' plates, and usually carry spines. The sides 
of the ray are occupied by the lateral or marginal plates (Fig. 1, 
which may be two in number or many ; and the upper portion by 
the dorsal plates, which are usually arranged in a more or less regular 
mesh work, and bear clustered spinelets or 'paxillse' (Fig. 1, p.) 
The internal cavity is extensive, and in it are disposed the digestive 
cDsca previously mentioned. The main radial nerve-cord is situated 
superficially to the water-vascular vessel, and its position is 
indicated in the diagram in red. 
It will not be necessary for the present purpose to go more 
minutely into the anatomy of the Starfish, we will therefore pro- 
ceed to compare a similar section made through the ray of an 
Ophiuran, which is given in Fig. 2. Here it will be observed that 
the outer walls are formed by four large plates, and what we see in 
the diagram are the ends of these plates which constitute the 
annuloid segments of the ray ; the whole space within, excepting 
only a very small cleft above and below, being occupied by the ex- 
tensions of a large calcareous disk, marked a., (Plate XV., Fig. 2), 
presenting various articulatory prominences and depressions, and 
which, when the upper arm -plate (Fig. 2, c.) is removed, will be 
seen to be the extremity of a vertebra-like joint, which articulates 
with a similar body that occupies the neighbouring segment. On 
