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POPULAR SCIENCE REVIEW. 
to cover the groove, the succeeding ones from opposite sides 
overlapping each other, while the tentacles are retracted. The 
holes, (s) represented in fig. 10 between the adjacent side-plates 
mark the positions of the dark spots or ‘ saceuli/ which occur 
at the sides of the food-grooves of nearly every Antedon , and are 
also to be found in the interior of the body. (PI. V. fig. 1. s.) 
These tentacles of the Feather-stars correspond to the 
tube-feet of a Starfish-arm, and contain side branches of a 
tube which is situated in the middle line of the arm beneath the 
food- groove, and is known as the water- vessel, or ambulacral 
vessel (PL Y. figs. 1, 2, w.v.) All the water-vessels of the 
different arms unite in succession as the food- grooves do, and 
converge to join the water- vascular ring, an annular tube, 
situated in the lip around the mouth.* From this ring a 
number of delicate tubules — the ‘ water-tubes ’ (w. t.) — hang- 
down into the body cavity, into which they open. It con- 
tains water which has entered it by the water-pores on the 
disc, so that the ambulacral system is indirectly in communica- 
tion with the water in which the animal lives. We are not 
yet quite clear about the real nature of the water-vascular 
system of the Echinoderms ; but it seems tolerably certain that 
it forms an important part of the breathing apparatus of the 
Crinoids, and that it contains some oxygen-carrying substance 
which is able, in the thin-walled tentacles, to exchange car- 
bonic acid for the oxygen of the surrounding water. 
Superficial to the water- vascular tube of each arm is a 
smaller tube, the radial blood-vessel ( b . v.) which is connected 
like its fellows with a blood- vascular ring around the mouth. 
Situated in the vertical axis of the disc is a lobulated organ 
known as the central plexus, from its consisting of a bundle of 
blood-vessels. (PL Y. fig. 1, c.j).) Some of these terminate 
above in the oral blood-vascular ring, while others extend 
outwards into the rays and arms and surround the genital 
glands. Others again give off side branches .which form a 
network over the digestive tube. Towards the bottom of the 
visceral disc the vessels of the central plexus group themselves 
into an inner set, surrounded by five outer ones, and so descend 
into the calyx. They pass through the central funnel between 
the inner ends of the first radials, at the bottom of which the 
five outer vessels corresponding in position with the radials 
expand considerably so as to form the ‘ chambered organ/ 
This consists of a central axis of minute vessels with five 
chambers (Pl. Y. fig. 1, ch.) clustered round it like the carpels 
of an orange. It is contained in a small, more or less basin- 
shaped plate, known as the centrodorsal piece. (Pl. Y. fig. 1. 
refers to these jlates. What he does describe, however, is something 
entirely different. 
* Compare Duncan, be. cit. pp. 347-351. 
