Hymenaster rex Perrier 1885

Hymenaster rex Perrier

Hymenaster rex Perrier, 1894:186, pl. 13: fig. 2.

The form of this species is more round than pentagonal, and inflated in life. It is of the consistency of a dense jellyfish; the hard parts are few and delicate. The five radial areas are well defined, and the broad interradial areas are without plates, spines, or calcareous deposits. Over the ambulacral areas, there are about 5–7 rows of delicate cruciform plates with long, thin, flattened lobes and a moderately high columnar pedicel. On top of the pedicel are 3 or 4 long, fine hyaline spines with expanded bases. On the carinals, these spines are about twice the length of the short pedicel, but both pedicel and spines increase in length from carinal to outer row. The lobes are in contact or slightly overlap those of the adjacent plates, and all abactinal plates are also in contact with the ambulacral and/or adambulacral plates. The spines support the dorsal membrane but do not protrude through it. I could find no spiraculae. The central osculum is large, and the webbed valves of 12–18 long, delicate glassy spines, as long as the longest abactinal paxillar spines, have an expanded base, each mounted on a small, triangular, flattened plate which in turn is borne on the upper ridge of the large, heavy, crescentic, oscular plate.

There are five of these plates forming a ring in the center of the abactinal surface proper, inside the nidamental chamber. The interradial madreporite, just outside this ring, is hemispherical and covered with deep gyri. The central stomach contains a few large, round, flat, delicate sieve plates. The gonads lie in clusters on either side of the ambulacrals, separated by a membranous interbrachial septum containing a few large, flattened, elongate or cruciform plates. The upper part of the ambulacral plates is about three times as long as wide and quite thin; they overlap. Below this broad, winglike flange, the plate narrows to a strut, connecting it with the flared outer or lower part of the plate that rests on the inner adambulacral plate. There are two sets of plates in the adambulacral position; the inner plate is by far the largest, cruciform to nearly square, with the lobes quite short except for the outer lobe (away from the groove) on a few of the proximal plates, which may be long, broad, and strong. The outer adambulacral plate rests on the actinal face of the inner adambulacral and bears three short, acute, conical spines on the furrow margin, and a small leaflike opercular plate on the actinal margin covering the small segmental pore. The inner adambulacral plate bears a single, long, slender, lateral spine which supports the actinal interbrachial membrane.

This spine has an expanded base and the tip is thicker than the midsection of the spine. There are 28–30 of these spines, the first 5 and the last 6 or 7 comparatively short; the rest are subequal. The broad mouth plates are nearly straight along the front, or apical, margin; each pair bears a stout pair of apical spines and a similar pair of epioral spines; they are thick, slightly flattened, and may be bifurcate at the tip. The margin also bears, to the side, 3 or 4 smaller acute spines. Interradially, between the mouth plates and resting on them, is a large round plate (odontophore) with an upward-extending flange. The ambulacral grooves are wide and there are two rows of large tube feet. The peristomial membrane is broad.

This specimen was badly damaged and poorly preserved; it was necessary to dry it to prevent further deterioration and to examine plates. During drying, large fatty globules were observed over the entire surface of the specimen.

While this specimen closely resembles others in the genus Hymenaster, it differs from the description of that genus in several important respects; there are no muscle fiber bands, no spiraculae, and the two series of adambulacral plates have not, to my knowledge, been observed in any other asteroid. I suspect that this may be the same species as that described by Perrier as Hymenaster rex; it would be impossible for him to have seen the double row of adambulacral plates without drying the specimen. The muscle fiber bands he describes may have been merely stretched strands of tissue. Although no spiraculae are present on the specimen before me, the very poor state of preservation may account for this; the dorsal membrane at the center of the disc had entirely disintegrated by the time the specimen was received.

This species has previously been reported only from the eastern Atlantic, in 400–800 fathoms.

MATERIAL EXAMINED.—Alaminos Station 14C/68-A-7 [R=81 mm, r=73 mm, Rr=1:1.1].