BRITTLE-STARS. 
3°7 
objects or rolled towards the mouth ; in the Streptophiurce, the faces of the ossicles 
have slight pits and processes, but none sufficient to prevent the ossicles being so 
twisted on their neighbours that the arms may be rolled up towards the mouth; in 
the Zygophiurce, the faces of the arm-ossicles have articulating knobs and pits, 
which prevent the arms from being rolled up towards the mouth. These vertebral 
arm-ossicles are encased in the tough outer skin of the arm, in which are developed 
granules, plates, and spines, which are least definite and regular in the Claclophiurcc , 
most definite in the Zygophiurce. The spines, which are clearly shown in the 
annexed figure of Ophiothrix, are borne on the side-plates of the arm, and aid the 
animal in locomotion. The integument of the disc also bears plates or scales of 
various sizes, often more or less covered 
with granules and minute spines. The 
precise arrangement of the plates on the 
top of the disc varies in different species; 
but five pairs of plates, known as the radial 
shields, are always present at the base of 
the arms, and are shown in the annexed 
figure. On either side of the arms where 
oin the disc, there is seen on the under 
surface a slit-like opening. These openings, 
known as the genital slits or clefts, are 
usually single but sometimes double; they 
lead into thin-walled pouches or bursae at 
the sides of the rays. In a living ophiurid, 
and contracts, 
and thus water is pumped into and out of 
the pouches, through the slits. The enter¬ 
ing water brings oxygen, which it ex¬ 
changes, through the thin walls of the 
for the carbonic acid contained in 
the water of the body-cavity, and then 
goes out again by the return current. 
Hence the pouches are called respiratory 
bursae. But they have another function, since the ovaries enter into them, and the 
ripe ova may either be carried out by the current through the slits, or they may 
remain and undergo direct development in the pouches themselves. Around the 
mouth are a number of short flat processes, or papillae, serving as strainers, and 
keeping foreign bodies that are not wanted for food from entering the stomach. 
Round the mouth are also twenty tentacles, which are really the modified tube-feet 
of the two first arm-segments of each arm. They are in a state of continual move¬ 
ment, assisting the food to enter, and clearing away the undigested residue, which 
is ejected from the mouth. 
The branched ophiurids, or Cladophiurce , are sedentary, attaching themselves 
by coiling their branching arms around corals and suchlike animals; but can move 
when they please. The same mode of life is also affected by a few of the simpler 
forms; but, as a rule, ophiurids have considerable powers of locomotion, of which 
the disc alternately expands 
