MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS, 183 
prolongationsof the body-cavity (figs.9—12). The middle one, 
into which open the water-pores on the arms and pinnules, 
is the genital canal already mentioned (figs. 9, 10, 12, 15, 
g.¢.). The canals above and below it, which communicate 
with one another at the end of each arm or pinnule, are 
known respectively as the ventral or subtentacular (s. ¢. c.), 
and the dorsal or ceeliac (c. ¢.). They are sometimes con- 
nected in the pinnules by a series of lateral trunks, as seen 
in fig. 12. In those Crinoids which have a central mouth 
(Antedon, Pentacrinus), the subtentacular canals all arise 
from a large central space in the axis of the visceral mass, 
around which the digestive tube is coiled.’ In Actinometra, 
however, which has an excentric mouth, the subtentacular 
canals of the disc gradually become indistinguishable from 
the general body-cavity, and there is no distinct axial 
celom. Currents proceed through the subtentacular canals 
to the tips of the arms and pinnules, and return to the disc 
again by the celiac canals (3). These currents are due to 
the action of cilia, which are not uniformly distributed, but 
are localised in little cups on the top of each pinnule joint 
that supports the lower part of the celiac canal (figs. 9, 11, 
12 ct. c.). The genital and celiac canals are continuous 
respectively with the ventral and dorsal portions of the cir- 
cumvisceral division of the body-cavity, 7.e. the space 
between what have been called the visceral and parietal 
layers of the peritoneum (3,8). In <Actinometra the sub- 
tentacular canals accompany the genital ones, and enter the 
ventral division of this space, the whole of which is laid 
open when the disc is removed from the calyx. 
The blood-vascular system of the Crinoids differs in several 
important points from that of the Starfishes and Ophiurids. 
In the first place there is no definite aboral ring, although 
there is a representative of the central plexus, the so-called 
“heart” of the Starfish. This organ consists of an irregular 
lobate bundle of vessels, which rises out of the calyx into 
the lower portion of the visceral mass. In Antedon and 
Pentacrinus it ascends nearly in the vertical axis of the body, 
in the immediate neighbourhood of the axial celom, to the 
point at which the latter subdivides into the five subtenta- 
cular canals of the disc. Its further course is exceedingly 
difficult to follow. Both Ludwig and myself believe it to be 
' This is very well shown in Ludwig’s diagrammatic vertical section 
of an Antedon (19, pl. xix). His figure (which is reproduced in the 
‘ Popular Science Review,’ July, 1880) may be advantageously compared 
with the two similar ones illustrating the Anatomy of Actizometra which 
accompany this paper (PJ. XII, figs. 14 and 15). 
