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Amphiplecta acrostoma Haeckel, 1887

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Amphiplecta acrostoma:
Cephalis subspherical, spiny, with sharp collar stricture on the base; its apical opening central, surrounded by a coronal of twelve to eighteen short spines. Length of the two joints = 7:8, breadth = 6:20. Thorax flatly conical, armed with bristle-shaped spines. Meshes of the network in both joints subregular, hexagonal, six to eight times as broad as the bars. Basal mouth bristly.

Dimensions ľ Cephalis 0.07 long, 0.06 broad; thorax 0.08 long, 0.2 broad.
Haeckel 1887
Benson, 1966, p. 464-467; pl. 32, fig. 2, (not fig. 1):

Amphiplecta cf. acrostoma Haeckel

?Amphiplecta acrostoma Haeckel, 1887, Challenger Rept., Zool., vol. l8, pp. 1223-1224, Pl. 97, fig. 10.

Cephalis long, cylindrical, relatively smooth, open at the top without a constriction, although a few specimens were observed with top partially or completely closed; upper margin of open cephalis irregular, with 6-10 or more upward divergent, thin, conical spines subequally spaced, of variable length (12-37 Ám); pores of cephalis large, subpolygonal to subcircular, equal to subequal, subregularly arranged in vertical rows, 4-6 on the half circumference of the cephalis. Cephalis separated from a low but broad conical thorax by an angular change in contour ventrally and laterally and by a distinct dorsal stricture, angular in outline in lateral view; the apical bar ascends dorsally from the median bar and is free in its proximal portion but distally is a cephalic rib which lies in the plane of the upper side of the angular, dorsal collar stricture; the bar extends as and is collinear with a short, thin, conical apical spine which originates from the junction of the dorsal collar stricture and the dorsal face of the cephalis. A short, thin, conical vertical spine extends from and is collinear with the vertical bar and originates at the base of the ventral cephalic face. Median bar short, cylindrical, located within the thoracic cavity below the separation of cephalis and thorax; with a short, conical axial thorn (2-6 Ám). Primary lateral bars prolonged as thin, conical spines that originate subterminally on the thorax below the collar region from short ribs in the wall of the thorax. The dorsal bar is free within the thoracic cavity for a short distance proximally but extends as a rib in the thoracic wall distally and terminates as a conical spine which arises from the basal margin of the thorax; the dorsal rib occupies a slight dorsal sagittal furrow in the thorax. Thorax smooth or with short, scattered thorns or spines, with large, unequal to subequal, irregularly shaped pores without regular arrangement. Basal margin of thorax irregular, with numerous spines representing intervening bars of an incompletely developed lattice.

Measurements; based on 6 specimens from stations 34, 81, and 93: length of cephalis 70-89 Ám, of thorax 68-80 Ám; breadth of cephalis 42-62 Ám, of thorax 123-188 Ám; length of apical spine 6-15 Ám, of vertical spine 5-10 Ám, of primary lateral spines 10-25 Ám, of dorsal spine 10-30 Ám.

Remarks. Haeckel's illustration of Amphiplecta acrostoma Haeckel (1887, Pl. 97, fig.10) is similar to the Gulf species in the presence of a tubular cephalis, open at the top and with large pores. The shape of the thorax and size and shape of its pores are similar in both Haeckel's species and the Gulf species. The corona of spines surrounding the open top of the cephalis and the numerous spines originating from the thorax of Haeckel's species are zig-zag whereas those of the Gulf specimens are straight. Whether or not this feature is an intraspecific variation cannot be determined until Haeckel's type material is examined. Haeckel (1887, p. 1223) defines Arnphiplecta as having the dorsal and primary lateral spines completely free within the thoracic cavity. His illustration of A. acrostoma, however, shows these spines as ribs in the wall of the thorax, thus not unlike the Gulf species. Except for the zig-zag nature of the cephalic and thoracic spines, A. acrostoma Haeckel does not differ significantly from the Gulf species.

Distribution. This species is very rare in the Gulf, but it is to some extent cosmopolitan because it is present as far north as station 184. It has a slightly greater frequency in the southern half of the Gulf; therefore, it has a greater affinity for normal oceanic waters than for Gulf waters. It is absent at stations 27, 64, 90, 99, 130, and those north of station 184. Its absence in the northern shelf region of the Gulf and at marginal localities indicates its preference for offshore, more nearly oceanic waters. Its distribution shows no evidence of response to upwelling.

Haeckel (1887, p. 1224) reports A. acrostoma from the North Pacific at "Challengerö station 244 at about 35 degrees N. latitude. It has not been reported elsewhere and, therefore, may be confined to tropical and temperate seas.
Benson 1966


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