258 PHYLUM ECHINODERMA. 
intestine between the stomach and the almost central dorsal 
anus two little outgrowths are given off, perhaps homologous 
with the “respiratory trees” of Holothurians (Fig. 139, 7.2). 
Some parts of the food canal are ciliated. 
The ccelom is distinct, though not much of it is left 
unoccupied either in the disc or in the arms. It is lined 
by ciliated epithelium, and contains a fluid with amceboid 
cells. A few of these have a pigment which probably aids 
in respiration; others are phagocytes, which get rid of 
injurious particles through the “skin-gills”; others con- 
tinue the work of digestion. 
When a starfish is crawling up the side of a rock, scores 
of tube-feet are protruded from the ventral groove of each 
arm; these become long and tense, and their sucker-like 
terminal discs are pressed against the hard surface. There 
they are fixed, and towards them the starfish is gently 
lifted. The protrusion is effected by the internal: injection 
of fluid into the tube-feet; the fixing is due to the pro- 
duction of a vacuum between the ends of the tube-feet 
and the rock. 
As to the course of the fluid, it is convenient to begin with the 
madreporic plate, which lies between the bases of two of the arms (the 
bivium). This plate is a complex calcareous sieve, with numerous 
perforating canals and external pores. It may be compared to the rose 
of a watering-can, but the holes are much more numerous, and lead 
into small canals, which converge into a main ciliated canal, the stone 
canal. This, as usual, opens into a ring canal around the mouth. 
The ring canal gives off nine glandular bodies (Tiedemann’s bodies), 
and, five radial tubes, one for each of the arms. Considerations of 
symmetry suggest that there should be ten glandular bodies, but in the 
inter-radius containing the stone canal there is only one. In many 
starfishes there are five or ten little reservoirs (Polian vesicles) opening 
into the circumoral ring, but in Asterzas rubens these are hardly dis- 
tinguishable from the first ampullee of the radial vessels. These run 
along the arms, and lie in the ambulacral groove beneath the shelter 
of the rafter-like ossicles. From them branches are given off to the 
bases of the tube-feet, but from each of these bases a canal ascends 
between each pair of ambulacral ossicles, and expands into an ampulla 
or reservoir on the dorsal or more internal side (see Fig. 134). The 
fluid in the system may pass from the radial vessels into the tube-feet, 
and from the tube-feet it can flow back, not into the radial vessel, but 
into the ampullz. There are muscles on the walls of the tube-feet, 
ampulla, and vessels. At the end of each arm there is a long unpaired 
tube-foot, which seems to act asa tactile tentacle, and has also olfactory 
significance. 
