MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS, 171 
(17, 21).1. The cell bodies are supported upon long rods, the 
bases of which are often forked, and rest upon the connec- 
tive-tissue membrane that separates the ambulacral epi- 
thelium from the vascular apparatus above it (fig. 5, ct.). 
Between these vertical rods lie the longitudinal fibres of the 
subepithelial layer (.), to which a nervous character is 
generally assigned (21, 33). Intercalated among the fibrils 
are small masses of nucleated protoplasm, which are con- 
tinuous with their substance, and sometimes mark the points 
atwhich they divide. Ludwig regards these small cells as 
nerve-cells, but they escaped the notice of Lange, who was 
consequently led to assign a nervous character to a cellular 
layer on the upper (?. e. dorsal) side of the above-mentioned 
connective-tissue membrane (fig. 6, ep.). 
The observations of Ludwig and Teuscher, however, prove 
that Lange’s masses of nerve-cells are merely local thicken- 
ings of the epithelium lining the radial perihzemal canals. 
They are not constant in all Asterids, and when present are 
not continuous through the ray. A similar thickening in 
the peristome of Asteracanthion rubens is represented in 
fig. 6, ep. 
Above and within the nerve band is the blood-vascular 
apparatus of the ambulacrum, which is very complicated in 
its arrangement. Between the basement membrane support- 
ing the ambulacral epithelium (fig. 5, ct) and the band of 
connective tissue that bears the lower transverse muscles of 
the arm, immediately under the water-vessel, is a relatively 
large space, which extends through the whole length of the 
ambulacrum. It is divided into two lateral portions by a 
perforated vertical septum, also longitudinal (fig. 5, v.s.), which 
supports the radial blood-vessel (fig. 3, d.). The latter, which 
is often somewhat plexiform in character, was more or less 
perfectly known to Hoffmann, Greeff, Lange, and Teuscher, 
but its true relations were first elucidated by Ludwig. The 
space already mentioned (fig. 3, 7.p.) between the water-vessel 
above and the ambulacral epithelium below, which is tra- 
versed by the perforated longitudinal septum, was named by 
Ludwig the “ perihemal canal.” It had been previously 
called the nerve-vessel or nerve-canal, and was supposed to 
form an integral part of the blood-vascular system.” Now, 
1 The numbers in brackets refer to a list of recent memoirs on Echino- 
derm anatomy, which is printed at the end of this paper. 
2 In his “Anatomy of the Invertebrata” Prof. Huxley speaks of the 
periheemal canal as the ambulacral neural canal, and expresses great doubt 
as to whether it really belongs to a special system of blood-vessels, The 
later observations ef Ludwig render the old view no longer tenable. 
