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Eucyrtidium hexagonatum Haeckel, 1887

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Eucyrtidium hexagonatum (p. 1489):
Shell smooth, ovate, with six to seven internal septal rings. All joints (except the first) nearly of the same length; the sixth joint is the broadest. The seventh and eighth joints (broken off in the specimen figured) gradually decrease toward the constricted mouth, which is as broad as the septum between the third and fourth joints. Cephalis and thorax together pear-shaped, with circular pores, sharp lumbar stricture, and a stout, pyramidal horn of the same length. The five or six abdominal joints with regular, hexagonal pores, six or seven in course of the length of each joint.

Dimensions.-Length of the shell (with eight joints) 0.2, length of each joint 0.025 to 0.03; greatest breadth (in the sixth joint) 0.1.
Haeckel 1887
Benson, 1966, p. 498-502; pl. 34, figs. 6-9:

Eusyringium siphonostoma Haeckel

Eusyringium siphonostoma Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1499, Pl. 80, fig. 14.
Eusyringium cannostoma Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1499, Pl. 80, fig. 13.

Large, broadly conical to spindle-shaped, generally smooth test, with 5-10 or more (generally 6-8) joints, all characterized by similar pores that are circular to polygonal, equal, and regularly arranged in longitudinal rows that are continuous between the joints, 25-35 or more rows on the half circumference of the broadest joint. In fully developed forms the last joint has a constricted, latticed, tubular mouth. All abdominal joints of nearly equal length in each test, separated by regular, circular, continuous, internal septal rings but generally not by constrictions; in a few tests the rings are irregular, and in some tests spirally arranged in part. Cephalis small, hemispherical, with a dorsal sagittal furrow occupied by the apical bar; separated from the thorax by an indistinct stricture. Apical horn stout, three-bladed, triangular, nearly centrally located on the top of the cephalis, absent or weakly developed in a few tests. Vertical spine short, extends within the interior of a short, inconspicuous, ventral tubule (text-fig. 26) which is present in several and perhaps in all tests. Four collar pores present. The dorsal and primary lateral bars extend as thoracic ribs; the ribs generally are inconspicuous and terminate in the wall of the thorax but extend as short, inconspicuous spines; in many specimens they are raised and are prolonged as low, latticed, wing-like structures which in several specimens terminate below the thorax on the first abdominal joint. Thorax campanulate, separated from first abdominal joint by an indistinct annular stricture and internal septal ring. Wall of test variable in thickness but generally thin.

Measurements; based on 30 specimens from stations 27 and 34: maximum length of test (including apical horn ) 125-295 m, maximum breadth 71-132 m.; length of cephalis 12-21 m, of thorax 12-37 m, of first abdominal joint 27-49 m, of remaining abdominal joints 18-49 m; breadth of cephalis 16-26 m, of thorax 31-68 m; length of apical horn 6-21 m, of vertical spine 0-4 m; diameter of constricted tubular mouth 37-57 m.

Remarks. This species is distinguished by its numerous but variable number of joints which are separated by generally regular, continuous, internal septal rings but not by constrictions and by its small equal pores arranged hexagonally in longitudinal rows. The few forms of this species that have their last joint complete are identical with Haeckel's illustrations of Eusyringium siphonostoma Haeckel and E. cannostoma Haeckel, e.g., with a constricted, latticed tubular mouth. The former has page priority over the latter and thus becomes the nominal species. Haeckel's two species differ only in the number of joints, a character variable within the species. The presence of latticed wings corresponding to the dorsal and primary lateral thoracic ribs is a character that is variable within the Gulf species; therefore, the lack of these features in the species mentioned above does not prohibit their being identified with the Gulf forms.
Several species differing only slightly from the Gulf species or representing incompletely developed specimens may be conspecific with E. siphonostoma Haeckel. Lithocampe trapeziana Mller (1859, pp. 42-43, Pl. 6, figs. 4, 5, 6) is apparently an incompletely developed form whose illustration lacks details for comparison with the Gulf forms. L. octocola Haeckel (1887, p. 1508, Pl. 79, fig. 6) has a constricted tubular mouth but lacks an apical horn; the horn in some of the Gulf specimens is either greatly reduced in size or not distinctly separated from the cephalis. Stichopilium rapaeformis Popofsky (1913, p. 404, text fig. 126) has a constricted, latticed, tubular mouth, as well as does S. thoracopterum (Haeckel) Popofsky (1913, pp. 401-403, text figs. 123-125), but Popofsky's sketches lack details. Eucyrtidium cienkowski Haeckel (1887, p. 1493, Pl. 80, fig. 9) is identical with incompletely developed forms from the Gulf.

Distribution. This species is one of the most abundant nasselline species in the Gulf. It is cosmopolitan in the Gulf and is absent only at stations 203 and 214. It is rare only at stations 27, 34, 46, 56, 64, 90, and 130 and common at all the others except station 206 where it is abundant (11.4%) and is the second most abundant species. It is the most abundant species at station 151 (8.6%). It is second in abundance at station 106 (4.4%), third at stations 133 (4.25%), 184 (5.8%), and 208 (8.6%), fourth at station 93 (3.6%), and fifth at stations 81 (3.6%), 91 (3.4%), and 191 (5.4%). Its low frequency in the southernmost portion of the Gulf (stations 27, 34, 46) indicates that it is rare but generally present in the eastern tropical Pacific. Its absence or near absence at marginal stations 56, 64, 90, and 130 suggests its preference for offshore, more nearly oceanic waters. Its high frequency at stations 71, 81, and 91 through 99 in the southern half of the Gulf may be the result of its response to upwelling along either coast. Its higher frequency throughout all of the northern half of the Gulf is partly the result of upwelling, but its common to abundant occurrence at stations not located within the diatomite facies or within or near areas of known upwelling, namely stations 184, 194, 206, and 208, suggests its tolerance of waters with higher than average temperature and salinity.
This species is apparently confined to tropical seas. Both Eusyringium siphonostoma Haeckel and E. cannostoma Haeckel were reported from the central Pacific at "Challenger" stations 272 and 271, respectively. The species mentioned in the remarks above are confined to tropical seas including the Mediterranean Sea. Further taxonomic study of this species and those closely related to it is needed before conclusions regarding its world-wide distribution can be made.
Benson 1966
Benson, 1983, p. 503:

Eucyrtidium hexagonatum Haeckel

Remarks. Nigrini (1967, p. 83) did not mention the inconspicuous vertical tube on the cephalis, which is characteristic of this species as well as of Eucyrtidium infundibulum and E.(?) hexastichum in the Gulf of California (Benson, 1966, p. 505, text-fig. 26).
Benson 1983


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