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Zoology Publications from Victoria University of Wellington—Nos. 42 to 46

Morphology

Morphology

External features: The Colony (Plate I, Fig. 1).

The colony is erect, arising from a flattened, root-like base from which is formed a strong stem. This stem branches irregularly forming large and small branches which, near the base of a colony, have the appearance of being formed of intertwined branches, and which may anastomose if they come into contact. Branching in small colonies (4-12cms high) is in one plane only, so that the colony has a flattened, fan-shaped appearance. Larger colonies have a more bushy appearance. The colour of the colony skeleton is an overall brown, but is paler on the small branch tips. The polyps (white in preservative) are borne on all branches, but do not arise directly from the tip of any of the branches. They are elongate (up to 2½ mm in length in the preserved state) naked cylindrical structures with a conical hypostome. The base of the polyp is without hydrophore or supporting bract of any kind. Tentacles are given off throughout the length of the polyp and normally appear irregularly placed, but in a well extended polyp are seen to be arranged in a spiral at intervals along the polyp, except for an oral whorl of four tentacles.

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Plate 1

Plate 1

Fig. 1 The Colony X 3/5. Fan shaped, and flattened in the plane of branching.

Fig. 2 Longitudinal section of a polyp to show 3 distinct regions: Hypostome; glandular region; region of large endodermal cells.

Fig. 3 Cross section of a polyp in the hypostome region.

Fig. 4 The skeleton of a branch tip. Flattened, longitudinal fibres are joined by struts (transverse fibres).

co, coelenteron; ect, ectoderm; end, endoderm; gl, gland-cell region; hyp, hpostome; long fib, longitudinal fibre; trans fib, transverse fibre; t, tentacle.

Plate 2

Plate 2

Fig. 1 The glandular region of a polyp. Cells packed with acidophil globules line the gastric cavity.

Fig. 2 Longitudinal section of a tentacle. It is capitate, and solid.

Figs. 3, 4 Cross section through a stem at a polyp base. The sections are 10μ apart. No skeletal supporting cup or hydrophore is present. The skeleton is lined with "inner" ectoderm which is continuous with the "outer" ectoderm. co, coelenteron; ect, ectoderm; ect', "outer" ectoderm; ect", "inner" ectoderm; end, endoderm; gl, gland cells; mes, mesoglea; n, nematocysts; sk, skeleton.

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Internal structure: The Polyp (Plate I, Figs. 2, 3; Plate 2, Figs. 1, 2).

In longitudinal section (Plate 1, Fig. 2) the polyp has three distinct regions, namely (a)—the hypostome, in which the main feature is the very strongly developed endothelial muscular cells; (b)—a secretory region of from ⅓ to ⅔ the length of the polyp consisting of many cells opening into the coelenteron and its branches [these cells are packed with acidophilic globules (Plate 2, Fig. 1)]; and (c)—at the base of the polyp there is a region of large vacuolated endodermal cells which extends for approximately a third of its length. The nuclei and cytoplasm of these cells are distally placed, and each cell usually subtends, at its base, two or three ectodermal cells.

The ectoderm of the polyp is somewhat cuboidal and is continuous with that over the surface of the whole colony. In extended polyps it is drawn out into a very thin layer (Plate 1).

The mesoglea is prominent in retracted zooids, but is thin when the zooids are extended. It is light staining, and appears structureless. In retracted polyps a very large number of cut ends of myonemes from the ectodermal cells are seen embedded in the mesoglea, and myonemes can be seen in the ectodermal cells.

The tentacles have a solid core of vacuolated endodermal cells and are capitate. Batteries of nematocysts occur in their swollen "cap-like" distal ends (Plate 2, Fig. 2). Nematocysts are sometimes seen in the ectoderm towards the base of a polyp, but apart from this are confined to the tentacles. Only stenoteles (penetrating nematocysts) have been seen, and they are usually about 5μ long, but often may be as large as 10μ long.

The Branch Tip (Plate 3, Figs. 1, 2, 3, 4).

Thicker branches of the colony give the appearance of being formed of intertwined smaller branches. The very tips of branches, although of small diameter show a similar structure when seen in transverse section.

In Plate 3, Figs. 1 and 3 are transverse sections through a typical branch tip and are 10μ apart. The stem or branch tip is seen to be composed of several coenosarc tubes. There is a central tube (that is endoderm, mesoglea, and ectoderm) and radially-placed coenosarc tubes.

Between these tubes are the skeletal elements. The skeleton is not formed around the whole circumference of the stem, but only between the inner ectoderm layer of adjacent coenosarc tubes (Plate 2). The whole stem is covered with a columnar epithelium.

The "inward facing" layer of the ectoderm of the outer coenosarc tubes, and the ectoderm of the central coenosarc tube are composed of vacuolate cells with the cytoplasm and nucleus distally placed. These ectodermal cells are clearly delimited from the endoderm of the tubes by a thin layer of structureless, lightly staining mesoglea. Small eosinophil particles are frequently present in the ectodermal cells, especially those cells which abut against the skeleton. The coelenteron of the coenosarc tubes appears as narrow branching channels (Plate 2). Gland cells packed with large eosinophil globules are seen in the endoderm of the tubes, especially in thicker stems, even though the coelenteron cannot be seen. In some transverse sections of a branch tip the coelenteron can be followed through the polyp to the endoderm of a coenosarc tube, but in the latter the coelenteron is not prominent. The "closing-over" of the coelenteron in the coenosarc tubes is probably due to contraction during preservation.

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Plate 3

Plate 3

Fig. 1 Cross section through a branch tip. Note the columnar ectoderm enclosing skeletal elements and endoderm. The regular arrangement of coenosarc tubes, and therefore of skeletal elements, is clearly seen.

Fig. 2 An enlarged portion of Fig. 1. Secretory ectodermal cells are seen at the junction of 3 ribs of skeleton.

Fig. 3 Cross section of a branch tip 10μ from Fig. 1.

Fig. 4 The area of Fig. 3 corresponding to the area shown in Fig. 2. The skeletal structures are reduced, but in their place are secretory ectoderm cells which show a granular cytoplasm.

ect, ectoderm; end, endoderm; p, polyp base; sec ect, secretory ectoderm; sk, skeleton.

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The ectodermal and endodermal cells are markedly different. Endodermal cells take stain more readily than do ectodermal cells; they also differ in the characteristics of their nuclei. Both the outer columnar and the inner secretory ectoderm cells have round nuclei of about 3μ. diameter, while endodermal nuclei may be up to 6μ diameter and are often irregular in outline. The nucleus of the ectodermal cells (Text Fig. 1b ) has a purplish staining nucleolus which is surrounded by about six small, darkly staining, chromatin bodies. From each of these small chromatin bodies there is a fine darkly staining radial process which connects with a small peripheral body. The nucleoplasm is irregularly granular. The nucleus of the endodermal cells (Text Fig 1a ) has a nucleolus which stains a brighter red, and fewer darkly staining bodies. But these bodies are larger than in the ectodermal cell nucleus, and are found irregularly scattered throughout the nucleoplasm. The nucleoplasm of the endodermal cell nucleus is much less granular than that of the ectodermal cell nucleus.

Bipolar and multipolar nerve cells occur between endoderm and ectoderm of the coenosarc tubes. The nucleus of these cells occupies the greater part of the cell body and stains very darkly. The cytoplasm is also deeply stained, and is agranular.

Although the outer ectoderm appears to be free of nematocysts, except at the base of a polyp, nematocysts occur in the internal etcoderm. In the latter cell layer, they total about 40% of the ectodermal cell number in thicker branches (Plate 2, Figs. 3 and 4). Both large (10μ) and small (5μ) stenoteles are present, and many stages of development can be seen.

Text Fig. 1

Text Fig. 1

Typical endodermal and secretory ectodermal cells. A, Endodermal cells. The cytoplasm contains large acidophil globules. The nucleus has a nucleolus and irregularly placed chromatin bodies. B, Secretory ectodermal cells. The cytoplasm is vacuolated. Fine chromatin strands radiate from the nucleolus to peripheral chromatin bodies in the nucleus.

chr, chromatin body; cy, cytoplasm; gl, globules; n, nucleus; nuc, nucleolus; st, chromatin strand.

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The Skeleton (Plate 1, Fig. 4; Plate 2, Fig. 3).

The skeleton of a branch tip is a network of "chitinous" fibres (Plate 1, Fig. 4). There are major fibres running longitudinally which curve to meet at the tip of the branch. These main fibres are joined by struts which are flattened in the direction of the branch and which occur about 0.09 mm apart. Combining the information given in the accompanying Plates, the skeleton of a branch tip can be described as a network cylinder surrounding the central coenosarc tube, with radially arranged, longitudinal strips of chitinous material which are flattened laterally, and joined to the central cylinder at intervals by transverse struts. These separate the peripheral coenosarc tubes from each other. In a well preserved specimen the whole skeleton is enclosed by soft tissue.

The skeletal fibres are of lamellar construction (Plate 2, Fig. 3). A branch tip skeleton had to be left in a 0.5% solution of lignin pink for 48 hours before it stained evenly throughout its length. In three pieces of skeleton taken in series from a branch tip, the colour deepened greatly away from the tip of the branch when they were all stained for ½ hour. The tip stained very faintly, while the furthermost piece stained deeply.