MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS. 177 
by Ludwig, who has shown that they correspond closely with 
the Asterids in the essential details of their organisation. 
Miiller’s discovery of the madreporic opening on one of 
the mouth shields of Ophioglypha lacertosa, which has been 
generally overlooked, has been confirmed by Ludwig and 
extended to other Ophiurids. There is usually only one 
pore which leads into a bent canal with lateral diverticula, 
and lined by ciliated epithelium. Some Ophiure and many 
Euryalids have several pores on one mouth shield, while in 
Trichaster elegans there is one pore in each interradius (22). 
Above the madreporite lie the stone-canal and central 
plexus, which are together enclosed in a perihemal space 
that, with its contents, was described as the stone-canal by 
Miller and Teuscher. The real stone-canal (water-tube), 
which was first recognised as such by Simroth, is lined asin 
the Asterids by a ciliated columnar epithelium, and it opens 
just above the madreporite into an ampulla-like cavity lined 
by pavement epithelium. Ludwig believes the pore canal of 
the madreporite to open into this space as its fellow does in 
the Asterids, but he has never been able to prove it, though 
he has demonstrated the connection of the upper end of the 
water-tube with the water-vascular ring (Pl. XI, fig. 4, w.r.). 
This bears either no Polian vesicles at all, or four, one for each 
of the remaining interradii (P.), or numerous blind tubular 
diverticula of various shapes, which are merely modified 
Polian vesicles (27, 29,32). In most cases the ring gives off 
bifurcating trunks which supply the large buccal feet (0,f’.). 
The blood-vascular system of the Ophiurids, like that of 
the Asterids, consists of dorsal and ventral rings united by 
a central plexus (Pl. XI, fig. 1). The former sends off 
branches to the genital glands (g. v.), while from the latter 
there arise the radial blood-vessels (figs. 1 and 4,0.). These 
last were discovered and correctly described by Lange, but 
Teuscher at first supposed them to be parts of the nervous 
system. They lie immediately above the nerve band (fig. 
4, n.), and send off branches to the tube feet which are 
accompanied by nerves. Between the blood-vessels and the 
water-vessels of the arms are the perihemal' canals of the 
former (fig. 4, 7. p.), which are connected laterally with the 
contracted remnants of the extension of the body-cavity 
into the arm that remain between the body wall and the 
vertebre (27, 32). 
In the disc the radial perihemal canal communicates as 
in the Asterids with the outer of the two perihemal ring. 
Neural canals, Huxley, 
