ECHINUS. oy 
minute calcareous corpuscles; (3) a layer of circular 
muscular fibres; (4) an inner ciliated epithelium continuous 
with that of the radial canal and the cavity of the tube- 
foot. 
The fluid contained in the system of canals, now 
described, is said to be sea-water, with traces of albumen. 
It is of a pale yellow or reddish colour; and, in addition 
to amceboid cells, corpuscles containing pigment are found 
floating init. The question of the origin of this fluid has 
been much debated. The view which meets with general 
acceptance is, that the cilia borne by the epithelial cells 
which line the pores of the madreporite and madreporic 
tube induce a current of sea-water to flow from the exterior 
through the madreporite; but an exactly opposite view 
has been maintained. Though locomotion is the primary 
function of the tube-feet, there can be little doubt that 
they are also concerned in respiration. An Echinus 
always retracts its tube-feet when the supply of respiratory 
oxygen falls short in the water in which it is kept; but if 
the water be changed, the opposite effect immediately 
occurs, the tube-feet being extended to their fullest 
capacity, and moved about actively in all directions. 
tt has been suggested* that the water-vascular system is 
morphologically and ontogenetically a left nephridium (see 
account of its development on p. 320), and that the current 
induced by the cilia of the madreporic pores and water- 
tube can be shown by experiment to be outwards, and not 
inwards, as described above. This view has not met with 
general acceptance and a series of experiments conducted 
by Ludwigt confirmed the statements of most previous 
observers with regard to the inward direction of the 
current. 
PRES om 
so 
* Ann. and Mag. Nat. Hist. Vol. XX. (1887), pp. 321—6. 
t Zool. Anzeig. XIII. (1890), pp. 377—9. 
