ECHINODERMA TA 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, (ii.) an intermediate connective 

 tissue layer with some muscle fibres, — this is the matrix for the 

 spines and plates, — and (iii.) 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, etc. 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. 131). 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. 1 3 1). 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 



