specimens, however, they seem to be wanting altogether on the ventral interradia, and
are confined to a few isolated spicules on the anterior and posterior extremities of the
dorsal area.
The spicules are wheel-shaped and slightly concave outwardly, varying from '2 to
*25 of a millim. in diameter. The spokes or radii, which are straight and slender, and
are united into a central boss, average about nineteen in number ; but they may vary
by three or four more or less.
The circumference or portion which represents the outer rim is divided by sutures
midway between each of the radial spokes ; and prolongations given off from the margin,
and bent sharply over at an acute angle, are directed- inward towards the centre, and
form a continuous series of equal, regular, elongate denticles. These denticles are
about one third of a radius in length, generally somewhat three-sided in form ; and their
number frequently exceeds that of the spokes of the wheel.
The manner in which the spicules are developed is interesting, and is easily followed
in some examples. The central boss is the first commencement, the margin of which
becomes crenulated ; and then denticles develop, which are gradually prolonged in the
form of spokes until the full size of the wheel is attained ; processes then form at the
extremities at right angles to the spoke, and these eventually meet and coalesce with
the corresponding processes from the neighbouring rays or spokes, whereby a continuous
rim is formed equal in thickness to the rays ; this rim next increases somewhat
in breadth, and the inwardly directed denticles begin to develop from the margin.
The tentacles are twelve in number, and, judging from their structure, are somewhat
hand-shaped, and would resemble when extended the,form which is found in Chirodota
(cf. Steenstrup, loc. cit.); when retracted, however, the upper portion is drawn within
the basal part or stem of the tentacle itself, the membrane forming a kind of hood
which arches over from the inward side, but leaves the extremities of the digitate prolongations
exposed and visible on the outer side.
Within the broad circlet of the tentacles stretches a muscular membrane, in the
centre of which the mouth-aperture is situated. The alimentary canal passes directly
from this to the posterior portion of the animal, where it is bent upon itself, then two
or three times sharply convoluted, and prolonged as far as the anterior third of the
body, when its course is again reversed, and it finally passes direct to the anal
aperture, this lower or intestinal portion being several times narrower than the rest of
the canal (see fig. 22). The various convolutions are held in position by fine reticulated
mesenteries.
The calcareous mouth-ring is very large and strong, and composed of ten ossicles ;
eight of these bear on their upperside a long, three-sided, wedge-shaped prolongation,
whilst the remaining two, which are situated on either side of the median dorsal ossicle
of the ring, have each two prolongations. Thus the prolongations correspond in number
with the tentacles, the bases of the latter being situated between them, whilst a fine
muscular band, which would seem to function as their retractor, stretches between the
prolongations halfway between the base and tip. A foramen for the passage of the
radial nerve is punctured through the prolongation, somewhat above its base.
The water-vascular ring is broad and conspicuous; and although only one Polian
vesicle is developed, which is large and heart-shaped, a number of dilatations occur at
intervals on the ring, which are suggestive of incipient or aborted vesicles (cf. fig. 24).
The generative organs consist of two groups of dichotomosing tubes which unite in
a single duct, that makes its exit.outsi.de the tentacular ring. The sexes are distinct
and readily distinguishable: in the female the tubes are shorter but very much broader,
and the large-sized ova can generally be seen within; in the male, on the other hand,
the tubes are longer and thinner as well as more numerous. In neither sex do these
tubes, as a rule, extend beyond the anterior third of the body.
Premature Form.—We are unable to speak from personal observation of the
growth-phases of Myriotrochus, as all the specimens at our disposal have been more or
less perfectly developed. Dr. Hj. Theel* mentions an individual 10 millims. in length,
presumably referable to this species, as the smallest he had examined. He states that
“ the skin contained a great quantity of the wheels in different stages of development,
some even perfectly formed, although amongst the whole it was scarcely possible to
find two that were identical. Many of the spicules had not more than six or ten radii,
but bore twice that number, or even more, of the denticles. This example, which was
a female, contained, notwithstanding its small size, fully developed eggs ” (loc. cit.).
Variations.—On the whole, Myriotrochus Rinkii is a species that seems to
undergo only very slight variation in form; the main difference which we have
remarked occurs in the number of spicules that are present on the cuticle. For
instance, in some examples the whole of the three dorsal interradia are closely filled
with spicules, and a fair sprinkling is distributed over the ventral area as well; whilst,
on the other hand, specimens are not rare in which these are confined to the median
dorsal interradial area, with, perhaps, just a few on the. edge of the adjoining areas,
the ventral portion being quite bare, except three or four isolated wheels at the anterior
extremity; and there are yet others even more sparingly provided, which have only a
few isolated spicules at the extremities of the dorsal surface, Respecting the above-
named variable character, we are bound to confess that we are at present unable to
assign even a locational value to these modifications, since both extremes occur together
in the material received from the most northern collecting-station recorded.
With these facts before us, it would seem that the characters upon which Sars
founded his genus Oligotrochus are all to be found in Myriotrochus; and consequently
no grounds exist on which that genus can be longer maintained. Indeed we have for
some time been of opinion that even the species 0. vitreus, Sars, could only be regarded
as a variety comprehended within the form of Myriotrochus Rimleii; but a recent paper
of Dr Theel’s, and a careful comparison of Sars’s description and figures with the
material we have had before us, lead to the conclusion that Oligotrochus vitreus may
probably rank as a species of Myriotrochus allied to M. Rinkii; and we are also further
inclined to think, from the description given, that Dr. Theel’s specimens from Novaya
Zemlya and the sea of Kara would accord more nearly with Myriotrochus mtreus than
* Nova Acta Reg. Soc. So. TJpsal. ser. iii. 1877, no. xvii. p. 6.
D