1875. Gtenodiscus comicidatus, Perrier, Stellérides du Muséum, p. 3 8 0 ; Arch, de Zool. exp. et gén.
U 300.
1879. Gtenodiscus comicidatus, Yiguier, Squelette des Steil., Arch, de Zool. exp. et gén. t. vii. p. 226.
Body depressed and goniodiscoid in outline ; radii five in number, with the arm-
angles well rounded ; proportion of greater to lesser diameter 2:1. The calcareous
elements of the abactinal surface form a compact network, similar to Astropecten, in
which the interspaces are but very small. A great number of small closely-placed
paxillæ are borne upon this framework, each carrying 5-10 round, blunt spinelets, the
whole so densely crowded together that the spinelets are normally directed upward
from the pedicle. The. abactinal surface is frequently puffed up and more or less
convex in profile (owing probably to the quantity of sand or clay with which this Starfish
fills its stomach) ; whilst a small peak-like protuberance rises from the centre,
around which the paxillæ rapidly diminish in size. The sides of the disk are perpendicular,
and formed of two series of marginal plates—one ventral, the other dorsal. Each
dorsal plate is ankylosed to a corresponding ventral plate—the pair thus formed being
separated from the neighbouring pair on either hand by a deep furrow, which follows the
lateral suture of the plates, the margins being fringed with a series of fine, compressed,
cilia-like spinelets, which arch over the furrow. Each of the dorso-lateral plates bears
a small compressed, but pointed, spinelet, which stands erect on its upper margin ; and
the lower or ventro-lateral series likewise carry a similar spinelet, which is placed near
the junction of the ventral and dorsal plates, and projects at right angles to the side
walls of the test. The last or terminal dorso-lateral plates of each side of a ray are
ankylosed together, and form a large arched or tubercular plate, indented on its outer
margin, and bearing three more or less prominent tuberclès—the rudiments of dorso-
piarginal spines. The furrows between the marginal plates are continued onto the
açtinal surface of the animal and extend to the ambulacral furrow, cutting up the
ventral interradial areas into band-like spaces, each of which is tessellated with irregular,
subquadrate, scale-like plates that imbricate upon one another, and form normally, in
large adult examples, a double alternating series behind each adambulacral plate. The
innermost band, however, of each area comprises two adambulacral plates ; and the
trapezoid tessellating scales, which here always form a regular double alternating series,
are, in consequence, twice as large in the neighbourhood of the furrow as the scales in
the other bands. All these plates bear on the margin that opens on the sutural furrow
a series of papillae that form a continuation with the papillae above-noted on the sides
of the lateral plates, from which they differ only in being not flattened, nor are they at
the same time so regular and closely placed. The adambulacral plate presents a wedge-
shaped projection into the furrow, and carries five or six papillae, three only of which
usually stand on the margin of the ambulacral furrow, the remaining two or three
(which are generally much smaller) being situated on the aboral margin opening on the
sutural furrow of the interbrachial area ; not unfrequently, however, one of them is as
large as the ambulacral spinelets, and is placed somewhat inward upon the plate, away
from the sutural fringe and behind the ambulacral series. Towards the extremity of
the ray the adambulacral plates stand next to the ventro-marginal plates, and are not
separated from them by the trapezoid imbricating scales above described.
Each pair of mouth-plates forms an ovoid mass, the inner or apposed margins of
the plates being elevated into a prominent keel. The innermost pair of mouth-papillae
are very large and thick, and taper to a point—the remainder, from 7 to 9' in number,
being considerably smaller and arranged round the free margin of the plate. Along,
or near to, the median keel of the mouth-plate are 8-5 coarse spinelets, the innermost
being large and thick, and are much less pointed than the marginal series. The
madreporiform body is frequently not more than its own diameter distant from the
margin, and is generally oval in outline and covered with elongate striae running in
the direction of the major diameter.
The entire body and all its appendages are covered with an investing leathery
skin.
Size.—Ordinary specimens are about 30—40 millims. in diameter, the largest
recorded by Sars, from Tromsö, being 65 millims.
Colour.—The colour is recorded as brown-red; specimens, preserved in spirit are
either black, greenish, or various shades of drab.
Habitat.—Gtenodiscus corniculatus is found in mud or soft clay bottoms at very
various depths, being dredged by Sars, at Finmark, in 40—200 fathoms depth; and
further south, at Christiansund, in 40-80 fms. It has also been taken in 25 fms. at
Arksut, by Barrett; and at Igaliko, in 60 fms., by Insp. Möller (iMtken),
Premature Form.—The young form of this species was described by Müller and
Troschel under the name of Ct. pgmceus. Small individuals, of about half an inch in
diameter, are characterized by the flatter test, the comparatively greater prominence of
the latero-dorsal spinelets, and the three large, conspicuously-developed spinelets which
are present on the terminal plate of the ray. The upper margin of this plate, which
lies towards the centre of the disk, is fringed with a series of papillae similar to those
on the sides of the lateral plates; and these papillae, as well as the spinelets, appear
to be subject to a greater or lesser degree of obliteration (or resorption) during the
progress of the growth of the Starfish; in fact in old specimens the spinelets become,
reduced to mere tubercles.
According to Lütken, the apical prominence in the centre of the disk is more prominent
and characteristically developed in young forms; but in the specimens which we
have examined (from Novaya Zemlya) it would appear to be quite the reverse, for we
have been unable to detect any difference, except a proportional diminution in size,
from the condition presented by the mature animal. The adambulacral plates in these
specimens bear their papillae or “ ambulacral spines ” on the furrow-margin, with one
large one placed thumb-like behind them on the aboral side.
At the extremity of the ray there are only two of the ambulacral spinelets; and
the thumb-like spinelet is larger than either of them, and is persistently present on every
plate. The mouth-plates are small and simple, having only three or four mouth-
papillae on the margin of each plate, and two only on the median ridge (or at most
three), the innermost of these being very large and prominent, and standing perpenp
2
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