FEATHER-STARS, RECENT AND FOSSIL. 
199 
the median groove of each arm, which is connected with an oral 
ring. This ring, and the bands connected with it, constitute a 
nervous system situated on the ventral side of the body.* A 
similar oral ring with radial prolongations occurs also in the 
Feather-Stars (PL Y. figs. 1, 2, n. v.). But it is altogether un- 
connected with the muscles, and has no influence whatever on 
the swimming movements. After the visceral mass has been 
turned out of the calyx altogether these movements go on just 
as well as before, while the movements of any arm are entirely 
independent of the ventral nerve of that arm ; for the un- 
grooved arms (PL YI. fig. 2), with no ventral nerve, swim just 
as well as the grooved arms with a ventral nerve, even after 
the latter has been divided. In some tropical Comatulce, half, 
or more than half, of the arms are in this ungrooved condition. 
While on the one hand the nervous nature of the ventral 
fibrillar band can hardly be denied, owing to its structural 
identity with the nerves of other Echinoderms, we must admit, 
on the other hand, that the very considerable locomotive powers 
of the Comatulce are localized in the chambered organ and its 
connections. These are altogether unrepresented in the other 
Echinoderms, but in spite of this, and of the singularity of 
their position, we cannot, with justice, refuse to consider them as 
nerves. Some day, perhaps, their presence will be understood. 
I Between the dorsal skeleton of the arms and pinnules, and 
the water- vessels on their ventral side, are three tubular pro- 
longations of the body- cavity. The middle one is known as the 
genital canal (Pl. Y. figs. 1, 2, g, c.), because it contains the 
genital gland (ay.) surrounded by a blood- vascular space, which 
is connected with the vessels of the central plexus. The canals 
above and below this one, are known respectively as the ventral 
or sub-tentacular (she.) and the dorsal or coeliac (c. c.). They 
communicate with one another at the end of each arm or pin- 
nule, and are connected in the body with different parts of the 
body-cavity, which is full of water containing the products of 
digestion. The sub-tentacular canal arises from a large central 
space (a. b.) in the axis of the body, around which the digestive 
canal is coiled, and a current proceeds through it to the tip of 
each arm and pinnule, which returns to the body by the coeliac 
canal. This current is produced by the vibration of cilia, which 
Rre not uniformly distributed, but localized in little cups on 
the top of each pinnule- joint (Pl. Y. fig. 2, ci. c.). 
The young of the Feather-Star leaves the egg as a little oval 
body about in length, shaped somewhat like a small barrel, 
and surrounded by four hoops of long vibratile cilia with a still 
longer tuft of them at its hinder end (woodcut a). By means 
of these cilia it swims about in the water. After a while, 
slender limestone rods make their appearance near the front 
* Compare Duncan, loc. cit. pp. 348-351. 
