172 P. HERBERT CARPENTER, 
however, it is regarded by Ludwig merely as a derivative of 
the body-cavity. At the intervals between the successive 
vertebrze its upper third is crossed by a series of transverse 
septa (fig. 5, ¢.s.). The longitudinal septum has a slight 
horizontal expansion at the level of the radial blood-vessel, 
which increases in size at the origin of each transverse 
septum, but never reaches the side of the perihzemal canal. 
Hence the latter is nowhere completely divided into three 
or four sets of chambers, as was formerly supposed. It gives 
off lateral extensions, which embrace the bases of the tube 
feet, and unite on their outer sides to form longitudinal canals 
(21). These, which were formerly regarded as lateral 
auxiliaries of the radial blood-vessels, are in connection with 
an extensive lacunar system in the body wall (fig. 3, dac.), 
which can be injected from the radial perihemal canal, 
and appears, like the latter, to be a derivative of the body- 
cavity, or perhaps, of the embryonic blastoceel. 
In the peristomial area of the disc the radial, nervous, 
and vascular trunks unite into their respective oral rings 
(Pl. XI, figs. 5,6, 8). The nerve-ring (#.7.) in the lip 
contains circular fibres packed among the rod-like bases of 
the epithelial cells. The basement membrane which sup- 
ports them also forms the wall of the perihzemal ring-canal,! 
into which there project the cellular masses described by 
Lange as nervous (fig. 6, ep.). The ring-canal itself is 
divided into two parts, an inner and an outer one, by an 
annular continuation of the longitudinal vertical septum in 
the radial canal (figs. 5, 6, 8, s.). This is situated rather 
obliquely, and supports the oral blood-vascular plexus (0. 0.). 
The inner canal (7. p.) is the oral blood-vascular ring of 
Tiedemann, while the outer one (0. p.) is the orange-coloured 
vessel described by him. ‘The latter is connected by inter- 
radial canalicular extensions with a widely-spread canal 
system, situated between the two layers which form the 
body wall, just as in the arms. This system is also con- 
nected, as will be seen later on, with the perihemal canals 
surrounding the genitai vessels (12, 21). Both the inner 
and the outer perihemal ring-canals have received various 
names from Greeff, Hoffmann, Teuscher, and Lange, who all 
regarded them, together with their radial extensions, as 
integral parts of the blood-vascular system. The real oral 
blood-vascular ring was seen by Tiedemann, Greeff, and 
Teuscher, but its true relations were only imperfectly 
known to them. Greeff conjectured what Ludwig has since 
1 This is the “circular neural canal’? mentioned in Huxley’s ‘ Inverte- 
brata.’ ‘ 
