178 P, HERBERT CARPENTER, 
canals, which has been hitherto described as the blood-vas- 
cular ring (fig. 4, 0. p.). The real oral ring, however, from 
which the radial vessels originate (0. 6.), is in close connec- 
tion with the nerve ring (m. 7.) and joins the upper end of 
the central plexus, which ascends from the mouth shield 
alongside the water-tube. As already mentioned, these two 
are enclosed within a perihzemal space which is connected 
with the inner perihemal ring-canal, just as is the case with 
the axial perihemal canal of the Asterids. Its other end 
opens into the perihzemal canal of the aboral ring (fig. 4, 
p-h.), the disposition of which is very singular. Although 
naturally belonging to the dorsal portion of the disc, only 
a part of it is to be found there, viz. those sections of the ring 
which lie beneath the radial shields (fig. 1, a. 05.; fig. 4, a. 6.). 
Resting on the mouth shields in the five interradial spaces 
are five other sections of the ring (a. 0,.), with one of which 
the central plexus is connected. The five radial and dorsal 
sections are connected with the five interradial and ventral 
ones by ten descending limbs, which pass downwards at the 
sides of the rays (fig. 1, a. 6,). Just before leaving their 
dorsal position these give off the ten principal genital vessels 
(fig. 1, g..), branches from which surround the glandular 
czeca situated between the genital clefts and the rays. The 
ceca, which are situated between every two clefts, receive 
their blood supply by lateral branches of the ten descending 
limbs at the sides of the rays (a. 6,). All the separate cca 
are connected by a cellular cord concealed within the vas- 
cular ring, which is regarded by Ludwig as a sterile portion 
of the generative apparatus comparable to the so-called 
rachis of the Crinoids. 
It has been hitherto supposed that the genital glands of 
the Ophiurids ‘pour their products into the peritoneal 
cavity, which communicates freely with the exterior by ver- 
tically elongated apertures placed interradially on its 
margins.” Ludwig’s observations, however, show that this 
view, which was advocated chiefly by Miller, does not 
altogether agree with the facts. He confirms the earlier 
discoveries of della Chiaje and Rathke, to the effect that 
the so-called genital clefts lead into a sac situated within 
the celom but not communicating with it, and receiving 
the ducts of the genital glands. There are ten of these 
“burs” in the disc, two in each interradius, which lie close 
alongside the radial skeleton (fig. I, 8). Hach is a thin- 
skinned invagination of the body wall, the adradial lower 
portion of which is supported by one of the so-called genital _ 
' Huxley’s ‘ Invertebrata,’ p. 504. 
