Acanthascus (Rhabdocalyptus) mollis (Schulze, 1886)
Original Description :
Shape and size: Moderately thin-walled, laterally compressed cup, 35 cm in height and 20 cm in breadth at the superior aperture. The diameter of the round compact stalk is about 6 cm. The large cup bears on its side a smaller, more elongated form, 15 cm in length, and 6 cm in width at its orifice; while beneath the latter there is a caecal protrusion or boss. The wall, which measures in its lower portion 4 to 6 mm in thickness, becomes gradually thinner towards the upper end, and terminates in an undulating or crisped, smooth, slender, oscular margin, without a fringe of spicules. At the lower portion of the cup, just above the solid, somewhat tuberculated base, there is a round hole, 5 mm in diameter, which establishes a communication between the gastral cavity and the water outside. The stalk is attached to the firm substratum by a slight basal expansion and includes here and there some intruded material, especially soil debris. The lower surface of the stalk, where it is fixed to the substratum, exhibits the familiar thin but firm reticulated plate, which is developed in all Hexactinellids at their point of attachment to foreign bodies.
A second smaller specimen of this species, measuring 13 cm in height and 8 in breadth, agrees closely with the above. It also exhibits a laterally compressed cup-like or saccular shape, with a thin oscular margin, but is attached to the firm substratum at several places, over a comparatively broad expansion.
Surface: The outer surface of the body is not quite uniformly curved, but exhibits gentle elevations and depressions, which may be, however, the results of desiccation. The inner surface of the wall of the cup exhibits here and there inconspicuous, ridge-like elevations with a thin edge. The extremely delicate dermal membrane, which forms a fine lattice-work, is preserved only in a few sheltered portions. The gastral skin, which forms an extremely delicate and fine network, exhibits a feltwork of spicular strands with meshes somewhat narrower than those in the dermal skeleton. The subsequent influence of damp has caused this specimen to fall in, so that the two halves of the wall of the cup have been united. I was therefore unable to discover anything definite as to the nature of the inner surface or of the oscular margin. Dr. Döderlein told me, however, that this specimen, even when still well preserved, exhibited a marked lateral compression. The inner wall or cup exhibited ridged elevations. The free upper margin had no marked plaiting, and ended in a. slender smooth edge without a fringe of spicules
Apertures: At the base of the cup the larger apertures of the efferent canals were apparent. In the smaller specimen two roundish apertures, 4 mm in diameter, occur near the lower end.
Skeletal elements: The parenchymal skeleton is represented by a feltwork of spicules, which are disposed in strands 2 to 4 m broad, and enclosing roundish spaces of various sizes The principal portion of the parenchymal skeleton consists of long slender diacts, occurring either isolated or disposed in strands. They exhibit a central nodal thickening, and the rough ends are sometimes conically pointed, sometimes simply rounded, and occasionally thickened in a club-shaped fashion. Between these there is a scattered occurrence of the familiar oxyhexasters, in which the principal rays are very short, and frequently almost aborted, while long terminal rays, present in variable number, are somewhat curved at their base, but otherwise quite straight on to the pointed outer end.
Oxyhexacts occasionally occur in which the rays are twisted at a point corresponding to that at which the terminal rays arise from the principals in the oxyhexasters. They are doubtless degenerate oxyhexasters, which are again almost reduced to simple oxyhexacts, retaining only a trace of their metamorphosis in the twisting at the base.
In certain regions, especially in the subdermal trabecular spaces, there is an abundant occurrence of large rosettes with terminal rays bearing terminal discs. These spicules but rarely exhibit the typical number of principal rays, but as a rule eight are present. When only six principals are present, they intersect as usual at right angles in a somewhat thickened node, and are rather thick and cylindrical. At the slightly expanded outer end they divide into three to six straight or somewhat S-shaped terminals, which diverge slightly in a tuft, and attain a length double that of the principals. Each terminal bears at its extremity a small, convex, transverse disc, with recurved marginal teeth. In these discohexasters the central node is occasionally much thickened, and provided in the angle between each two principal rays with radial, tubercle-like rounded processes, winch may also be drawn out into simple spines. In other cases, one or two of the principals are especially thick, and split up externally into several terminals; and this modification appears to me to indicate the way in which the numerous rosettes have arisen, which bear eight principal rays arising at approximately equal angles from the central node. The variations in the rays of these rosettes are so numerous that it is impossible to attempt to describe all the modifications. I shall only note that not unfrequently the central node becomes swollen into a conspicuous sphere, from the surface of which, besides several broad principals, numerous terminals also arise, evidently by the basal splitting of the principals. The splitting of a particular portion may thus increase till the whole principal is divided.
Under the skin these peculiar discohexasters occur in abundance, but in other regions, and especially in the subgastral trabecular space, peculiarly modified oxyhexasters, oxyhexacts, and remarkable diacts, derived by reduction from the latter, occur abundantly. In numerous oxyhexasters and oxyhexacts, the curved basal portion of the otherwise quite straight, gradually pointed rays, bears a coating of fine spines or barbs, directed obliquely inwards. These barbs are very numerous, and sometimes so long that those of adjacent rays almost unite. On these spinous oxyhexacts the rays are sometimes curved, and this not unfrequently takes the form of a spiral twisting of the two rays on the same axis. If it happens, as is by no means unfrequent in these spinous spicules, that the number of rays is reduced, forms result in which a spherical, spinous, central body bears at its two opposite poles two spinous rays, which are twisted half round in a spiral and then continued in a straight course to end in a simple point, or to be divided into several pointed terminals. But the multiplicity of structure in these apparently reduced forms is so extremely great, that I will not begin to give a detailed account of the multitudinous modifications.
The dermal skeleton is supported by medium-sized hypodermal oxypentacts, in which the long smooth rays sometimes exhibit a simple curvature, but are, as a rule, quite straight. In the dermal membrane itself, numerous autodermal diacts occur, in which the rough rays, lying in one axis, end conically or are somewhat rounded off. The centre usually exhibits two or four projecting tubercles, but these are in other cases entirely absent. There is an isolated occurrence of well developed tetracts with rays crossed at right angles, and even pentacts with a ray penetrating the parenchyma and resembling that of the diacts. Monacts occur less frequently than in Hyalascus baculifer, but still in tolerable abundance. They may be with, or without lateral tubercles at the expanded end. Finally, I should mention that here and there small discohexasters occur with numerous terminal rays on the expanded ends of the principal rays, like the similar structures in Acanthascus (Staurocalyptus) roeperi (Schulze, 1887).
The gastral skeleton consists of strong, rough oxyhexacts, similar to those in the genus Bathydorus.
Synonymised taxa : Acanthascus mollis (Schulze, 1886) (subgenus assignment)
Rhabdocalyptus mollis Schulze, 1886 (genus transfer)
Links and Resources
World Porifera Database : http://www.marinespecies.org/porifera/porifera.php?p=taxdetails&id=171983