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Stichopilium bicorne
Haeckel, 1887
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Description - Add description
Benson, 1966, p. 422-424; pl. 29, figs. 1-2:
Stichopilium bicorne Haeckel
Stichopilium bicorne Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1437, Pl. 77, fig. 9.
Cephalis smooth, cap-shaped, separated from thorax by a change in contour, but in most specimens the apical-lateral arches and the ventral arch are represented by ribs which occupy slight furrows separating the cephalis from small, dorsal and ventral lobate swellings of the proximal portion of the thorax; pores of cephalis small, equal, subcircular to polygonal, regularly arranged. Two prominent, straight, nearly equal, three-bladed cephalic horns lying in the sagittal plane correspond to the dorsally ascending apical horn and ventrally ascending vertical horn. The former extends from the apical bar which in its upper portion above the junction with the apical-lateral arches is a dorsal rib in the wall of the cephalis and in its lower portion is free within the cephalic cavity. The latter horn extends from and is collinear with the vertical bar which joins with the ventral arch. The dorsal and primary lateral bars extend as ribs in the thoracic wall and are prolonged into heavy, equal, three-bladed, wing-like spines which are generally straight and diverge downward but are nearly horizontal in a few specimens; spines latticed proximally in a few tests. Thorax pyramidal with slightly concave sides between the thoracic ribs in its proximal portion; its distal portion circular in section, campanulate or inflated cylindrical; the wall of the thorax is extended outward where each of the three basal spines originate, thus giving the appearance of three latticed wings originating from the middle portion of the thorax. Number of abdominal segments variable from none to at least two; these joints separated from one another and the thorax by distinct constrictions which are generally occupied by continuous internal septal rings. Abdominal joints cylindrical, inflated, truncate conical or campanulate. Surface of thorax and abdominal joints smooth. Pores of these joints similar, of equal size, small, hexagonal to subcircular, hexagonally arranged in transverse rows.
Measurements; based on 10 specimens from stations 27, 34, 46, 60, and 64: length of test 98-193 µm, maximum breadth (thoracic or abdominal joints) 80-138 µm; length of cephalis 21-25 µm, of thorax 62-102 µm, of first abdominal joint 25-49 µm; breadth of cephalis 20-31 µm, of thorax (distal portion) 83-111 µm; length of apical horn 14-49 µm, of vertical horn 15-37 µm, of basal spines 12-53 µm.
Remarks. There is no doubt that the Gulf species is identical with Stichopilium bicorne Haeckel. Campbell (1954, p. D136) states that Stichopilium Haeckel (1882, p. 439) is an objective synonym of Triacartus Haeckel (1882, p. 437) and that the type species of the latter is Stichopilium cortina Haeckel (1887, p. 1437), an unfigured species. Earlier, however, Frizzell and Middour (1951) designated S. bicorne as the type species of Stichopilium. I agree with the latter designation as well as with Haeckel's (1887, p. 1436) first revision in which Triacartus is placed in synonymy with Stichopilium, the former becoming a subgenus under the latter.
Distribution. This species is cosmopolitan but very rare in the Gulf. It is absent at stations 90, 95, 99, 130, 191, 194, and all those to the north. Its absence at most of the marginal localities indicates its preference for more nearly oceanic offshore waters. Its very rare occurrence indicates that it does not respond significantly to upwelling in the Gulf.
Haeckel (1887, p. 1437) reported this species from the central Pacific at "Challenger" station 271. Its absence at high latitudes indicates that it is a tropical species.
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