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Actinomma trinacrium Haeckel, 1860

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Actinomma trinacrium (p. 254):
Cortical shell as well as both medullary shells thin walled, with subregular, circular pores, twice as broad as the bars. Radial proportion of the three spheres = 1: 3: 9; radial beams between them twenty, prolonged outside into strong, three-sided pyramidal spines, as long as the radius; between them, on the surface, numerous similar spines. Sometimes the latter remain smaller, the pores more irregular, and then this species corresponds to Echinomma trinacrium.

Diameter of the outer shell 0.09, middle 0.03, inner 0.01; cortical pores 0.008, bars 0.004; length of the spines 0.04, basal breadth 0.01.

Habitat: Mediterranean (Messina, Corfu, Haeckel), surfaces.
Haeckel 1887
Echinomma trinacrium (p. 258):
Cortical shell thin walled, roundish pores, twice to three times as broad as the bars. Both medullary shells with regular, circular pores. Radial proportion of the three spheres 1:3:9. On the surface about twenty three-sided pyramidal main spines, as long as the radius, and numerous (forty to sixty) by-spines of half length. (Compare with this species Actinomma trinacrium, with which it is connected by transitional forms).

Diameter of the outer shell 0.09, middle 0.03, inner 0.01; cortical pores 0.008 to 0.012, bars 0.004; length of the spines 0.02 to 0.05, basal breadth 0.01.

Habitat: Mediterranean (Messina).
Haeckel 1887
Echinomma trinacrium (p. 116):
The forms which I have tabulated under this name are somewhat uncertain. They are distinguished from the forgoing species by a strong, rather think outer shell, more numerous and stronger byspines, as well as by a different construction of the inmost shell, which is in most respects like the middle one.

There is nevertheless on the one side a considerable agreement with Chromyechinus borealis, only that the outside shell (the fourth) is wanting, on the other side a considerable resemblance to younger forms of Drymyomma elegans, where the characteristic branched byspines are still wanting or are branchless needles. To this must be added that different forms of E. leptodermum may also have rather strong outer shells and more numerous spines.

As a rule, though, the forms of Chromyechinus borealis may easily be recognized by the transverse processes on the radial spines, which here, as in E. leptodermum and Hexacontium enthacanthum suggest the beginnings of the still undeveloped outer shell. The forms of Drymyomma elegans have characteristic long slender main spines and narrow byspines. Yet, I am not sure if there be not still another species, most nearly answering to E. trinacrium, but with long, narrow principal spines and byspines, the latter always being branchless.

It is difficult to examine the inmost shell. I have not yet succeeded in ascertaining with certainty whether such a firmly constructed inner shell with which E. trinacrium is depicted by HAECKEL (1862, pl. 24, f. 6-8) is also characteristic of Chromyechinus borealis and Drymyomma elegans. I have, however, seen such a shell.

As I have already suggested in a previous paper (JÖRGENSEN, 1900, p. 57) it is not impossible that what I have called E. trinacrium may be certain young forms of Chromyechinus borealis, where the above mentioned transverse processus on the radial spines are wanting. On the other hand, it is just as likely that there may be with us another species differing from both Drymyomma elegans and Echinomma trinacrium, to which the supposed intermediate forms belong. This species would be comparatively frequent on the northern coast of Norway.

Until this is made evident, it will be best to keep to the species which are always easily recognized, viz. Echinomma leptodermum, Chromyechinus borealis and Drymyomma elegans.

Occurs in the plankton like the foregoing species.
Jørgensen 1905
Actinomma trinacria:
Haeckel (1887) described two species Actinomma trinacrium (recte trinacria) and Echinomma trinacrium (recte trinacria), and we will quote his descriptions.

Actinomma trinacrium: “Cortical shell as well as both medullary shells thin walled, with subregular, circular pores, twice as broad as the bars. Radial proportion of the three spheres = 1:3:9; radial beams between them twenty, prolonged outside into strong, three-sided pyramidal spines, as long as the radius; between them, on the surface, numerous similar spines. Sometimes the latter remain smaller, the pores more irregular, and then this species corresponds to Echinomma trinacrium”.

Echinomma trinacrium: “Cortical shell thin walled, roundish pores, twice to three times as broad as the bars. Both medullary shells with regular, circular pores. Radial proportion of the three spheres 1:3:9. On the surface about twenty three-sided pyramidal main spines, as long as the radius, and numerous (forty to sixty) by-spines of half length. (Compare with this species Actinomma trinacrium, with which it is connected by transitional forms)”.

In our material both forms are present, but the majority are transitional forms, and it is not possible to assign these forms to either Actinomma trinacria or Echinomma trinacria. We regard Echinomma trinacria as a junior subjective synonym of Actinomma trinacria, since this name has priority according to the rules of ICZN.

As we already have pointed out, the juvenile stage of the long-spined form of Actinomma boreale, when only three shells have developed, strongly resembles A. trinacria. Very few typical adult and well developed A. trinacria have been observed, while A. boreale is rather common. We therefore assume that most of our transitional forms should be classified as A. boreale, which was also suggested by Jørgensen (1905). Furthermore, he stated: “As a rule, though, the forms of Cromyechinus borealis may easily be recognized by the transverse processes on the radial spines, which here, as in E. leptodermum, suggest the beginning of the still undeveloped outer shell. The forms of Drymyomma elegans have characteristic long slender main spines and narrow byspines. Yet, I am not sure if there be not still another species, most nearly answering to E. trinacrium, but with long, narrow principal spines and byspines, the latter always being branchless”.

Distribution - To be found at the west coast of Norway in the deeper water layers (Schröder, 1909). Fairly common, but never in great numbers. Also found in the Mediterranean Sea (Haeckel, 1887).
Cortese and Bjørklund 1998


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