MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS, 187 
pheral vessels expand slightly in each nodal joint, and each 
gives off one cirrhus vessel; hence, every nodal joint con- 
tains, as it were, a small edition of the chambered organ 
situated in the calyx, which was considered as a ‘heart ” 
by Miiller, and is apparently still so regarded by Greeff (13, 
14). According to this view, therefore, there must be a 
“heart” at every node of the stem of Pentacrinus, the 
length of which is sometimes very considerable. In the 
fossil P. briareus, for example, with its 50—70 feet of stem, 
the number of these ‘ hearts ” must have been rather large. 
The axial cords enclosing the cirrhus vessels of Comatula 
(figs. 14, 15, cz. v.) are given off from the lower or dorsal 
portion of the fibrillar envelope of the chambered organ. 
From each of its ventral interradial angles a large cord 
passes upwards and outwards (figs. 13, 14), and forks almost 
immediately. The right branch of one fork, and the left 
branch of its neighbour, enter two adjacent openings on the 
inner face of each first radial. They run side by side 
through its central canal and on into the third radial, where 
each of them forks (fig. 13). The two right branches enter 
the central canal of the skeleton of the right arm, while the 
left branches enter that of the left arm, to form their respec- 
tive axial cords. Before leaving the third radial, however, 
these two cords are united by a transverse commissure. 
There are also commissures in the first radials. The two 
cords which each contains are united with one another and 
with those of adjacent radials by one continuous circular 
commissure (figs. 14, 15, c. co.), lodged like the other cords 
in special canals. 
It is very difficult to determine whether these cords 
enclose vessels as the axial cords of the cirrhi do. Greeff 
(14) has attempted injection but without satisfactory results. 
The appearances sometimes presented by transverse sections 
seem to indicate that each cord consists of two lateral 
fibrillar masses enclosing a central structure, which may 
possibly be a vessel, though I have never been able to satisfy 
myself that it has a lumen. This may, however, be the 
result of a post-mortem contraction, while both Ludwig and 
myself have met with coagulum, not only in the plexiform 
tissue forming the organic basis of the skeleton, but also in 
the substance of the axial cords themselves. This rather 
tends to support Ludwig’s idea that whether the cords con- 
tain distinct vessels or not, they do serve as the medium by 
which a nutritive fluid is able to enter the substance of the 
skeleton. 
Histologically, these cords have a remarkable resemblance 
