ECHINODERMATA 227 
on the skin or penetrating through the branchiae into the 
coelom. 
On the dorsal surface of the disk, situated interradially, lies 
the madreporic plate, through which the water-vascular system 
communicates with the exterior. The two arms which lie right 
and left of this plate are termed the “ bivium,” and contrasted 
with the other three or “ trivium”; in mapping out the various 
organs of the body these will be found to be convenient terms. 
The anus lies near the centre of the dorsal surface of the 
disk. 
The skin is formed of (i.) an outer cylindrical epithelium 
with nerve fibrils at the base, (i1.) an intermediate connective 
tissue layer with some muscle fibres,—this is the matrix for the 
spines and plates,—and (111.) an inner coelomic epithelium, which 
is ciliated; this last lines the true coelom or enterocoel, a 
spacious cavity containing the alimentary canal, the generative 
organs, ete. The coelom contains a fluid in which amoeboid 
corpuscles float. 
The angle which the two series of ambulacral plates in each 
arm make with one another is floored in by the outer layer of 
the integument, the nerve plexus of which is thickened and 
forms the radial nerve (Fig. 151). The cavity thus formed is 
the radial perihaemal vessel or blood-vessel of French authors ; 
it is divided into a right and a left portion by the presence of 
a median mesentery. This mesentery in this species, but not 
in others, has a certain amount of glandular tissue in it, which 
Ludwig describes as a blood-vessel. 
On the dorsal surface of the starfish numerous delicate 
processes of the skin may be seen projecting above the general 
level of the body-wall. These thin-walled extensions of the 
integument are known as dermal branchiae; the coelomic fluid 
passes freely into them, and they doubtless serve as respiratory 
organs (Fig. 151). It has been recently shown that some of the 
amoeboid cells of the coelomic fluid (phagocytes), when they 
have eaten any particles which it is desirable should be ejected 
from the body, make their way to the walls of these dermal 
branchiae, and force a passage through them to the exterior, 
whence they are washed away. 
Besides the enterocoelic ciliated body-cavity, there are a 
