MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS, 18] 
3. Crinoidea. 
The Crinoids occupy a very curious position among the 
Echinoderms. There are certain structural peculiarities in 
which they resemble the Starfishes much more than the 
Holothurians do, and yet they possess a complicated system 
of organs which is absolutely without parallel in the other 
Echinoderms. 
The ambulacral epithelium lining the food grooves is 
essentially similar to that in the ambulacra of the Star- 
fishes. It consists of closely packed columnar cells, each 
with a cuticle and numerous cilia. Their lower ends are 
pointed and are apparently connected with some vertical 
fibres, among which lie the longitudinal fibrillar bundles of 
the ambulacral nerve. These vertical fibres, which arise 
from the thin membrane separating the nerve and radial 
blood-vessel, are probably merely of a connective-tissue 
character. Sometimes in Antedon Eschrichtw there is a 
second connective-tissue lamella which separates the nerve 
from the epithelium above it, and supports the lower ends 
of the epithelium cells (Pl. XII, fig. 10, 7.), but its presence 
does not appear to be constant. 
Intercalated among the longitudinal fibrils are many very 
minute cells, just as in the ambulacral nerves of the Star- 
fishes and Ophiurids, which have a close histological resem- 
blance to the sub-epithelial bands of the Crinoids. Lateral 
branches proceed from the ambulacral nerve to the succes- 
sive tentacular groups, but there are no representatives of 
the muscle nerves which occur in the Ophiurids. 
Between the water-vessel and the ambulacral nerve the 
middle line of the arm (or pinnule) is occupied by the radial 
blood-vessel, just as is the case in the Starfishes (Pl. XII, 
figs. 9,10, 11, 0.), but it is not enclosed in any perihemal 
space, and is usually of very small size, so as occasionally to 
escape notice altogether. On the other hand, it is sometimes 
relatively very large, asin Actinometra nigra. Immediately 
beneath it is the radial water-vessel (figs. 9—11, w.), from 
which lateral branches proceed alternately on opposite sides 
of the arm to the different tentacular groups, each branch 
being accompanied by corresponding ones from the blood- 
vessel and ambulacral nerve. The three radial trunks are 
continued over the disc (figs. 14, 15) to the peristome, where 
they unite into their respective circumoral rings. 
Depending from the water-vascular ring into the body- 
cavity are a number of small tubules, open below and lined 
by ciliated columnar epithelium (fig. 14, w. ¢.). Their number 
