ECHINUS. oa 
which have been described as radiating from it, and 
traversing the ambulacra between the water-vascular and 
pseudhemal canals. Such vessels are not evident in 
carefully prepared serial sections of the ambulacra of the 
present species. 
In connection with the blood-vascular system, the 
central plexus or axial organ may be described. It is a 
fusiform body (Pl. III., figs. 24 and 25, cen.pl.), consisting 
of a network of connective tissue, which almost com- 
pletely fills the cavity of the axial sinus (PI. IIL, figs. 23, 
feeande2o; Pl y., fig. 37, ax.sin.), to the inner wall 
of which it is attached by connective strands. Its lower, 
oral end rests upon the circum-cesophageal vascular ring 
(fig. 23), and is said to open into the blood-vascular ring 
by those authors who maintain the existence of such a 
structure. Its upper apical end is attached to and per- 
forates the septum which divides the axial sinus from the 
madreporic ampulla (Pl. IIl., fig. 24). The reticulum 
of connective tissue is densest at the periphery of the 
bream el Ell. fic, 25; Pl. IV., fig. 35)..-Im the deeper 
cortical portion (fig. 36) the network is much more open, 
and its fibres more clearly defined. Numbers of nucleated 
cells of rounded form (fig. 35), from which lymph corpuscles 
are said to be derived, lie within the meshes of the 
reticulum. The central plexus has given rise to more 
discussion than any other structure found in the Echin- 
oderm body; and the names “heart,” ‘‘kidney,’’ “ plasti- 
dogenic organ,” “lymph gland,” indicate the functions 
which have been ascribed to it by various authors. 
In addition to the peripharyngeal or lantern sinus and 
the axial sinus, the coelom is further sub-divided by a 
circular membrane, which connects the anal end of the 
rectum with the surrounding body-wall, and encloses a 
small perianal sinus. Below this, another similar mem- 
