Discussion

The fact that the skeleton of the Solanderiidae is internal and is completely covered by a layer of epithelial cells in the living animal has undoubtedly led to the earlier belief that the skeleton is mesogleal in origin. However, it is evident from this study that each branch of a colony is composed of several coenosarc tubes. The appearance given in cross sections of a branch tip (Pl. 3, Figs. 1 and 3) is very similar to that of a cable containing many wires, cut in cross section. Each of the coenosarc tubes consists of endoderm, mesoglea, and ectoderm, and therefore could be expected to be capable of producing a perisarc, just as are other skelton-bearing hydroids.

The outer epithelium of the colony probably is formed by fusion of the superficial layer of the ectoderm of the peripheral coenosarc tubes in each branch. This idea is given weight by the fact that the outer epithelium can be seen to be continuous with "inner" ectoderm of the coenosarc tubes, especially at polyp bases (Pl. 2, Figs. 3 and 4).

The "inward facing" ectoderm of peripheral coenosarc tubes and the ectoderm of the other coenosarc tubes contained in a stem apparently become modified into a secretory tissue which forms the skeletal fibres. Skeletal material is secreted only where the ectoderm of two or more coenosarc tubes make contact. The skeleton is thus a "perisarc" contributed to by two or more coenosarc tubes. The cross sections shown in Plate 3 support this interpretation, and there is no doubt that the skeleton is ectodermal in origin. The skeleton may differ in composition in young and older parts of the stem. Lignin pink, which is considered a specific stain for chitin, does not stain the skeleton of branch tips as readily as it does the skeleton from thicker parts of the stem.

The method of growth of the colony can be seen from serial sections of a branch tip. From the tip to the thicker parts of a branch there is a central coenosarc tube, with peripheral tubes which anastomise with the central tube and with each other (but the central tube anastomoses far more frequently with the peripheral tubes than do the latter with each other). The polyps communicate with more than one coenosarc tube at their base (Plate 3, Fig. 1), and this base may be very large and extend over two or more skeletal "holes". (Plate 2, Figs. 3 and 4).

A serial reconstruction of the soft parts (endoderm only) (Text Fig. 2) shows these and other features of a stem tip. The central coenosarc tube has no terminal polyp. Some peripheral tubes may have a terminal polyp, but others do not. This type of anastomosing of coenosarc tubes is intermediate between that described for Clathrozoon wilsoni and Plumularia procumbens by Spencer in 1890, although there are fewer tubes in any one branch tip.

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The skeleton shown in Text Fig. 2 was not reconstructed from serial sections. Skeletal rods were drawn between the peripheral endoderm tubes and then joined through the holes between the anastomosing tubes. This showed a skeleton of a branch tip similar to that described from serial cross sections as illustrated in Plate 3.

The stem is clearly a fasicled or bundled structure. But the status of each of the elements composing it is more difficult to define. The alternative possibilities are firstly, that the central coenosarc tube is equivalent to a hydrocaulus and is surrounded by its branches and stolons, which anastomose with it and with each other, and secondly, that the whole stem is a rhizocaulome, being composed of upright, branching and joining stolons.

Two features indicate that the first alternative is the more likely because (a) the stem tip clearly has a central coenosarc tube, and (b) the fact that this central tube anastomoses much more frequently with the peripheral tubes than the peripheral ones do with themselves. This suggests that the central tube is a primary structure, and the others are secondary to it.

The method of growth seems to be "secondary monopodial" (Hyman 1940, p. 406) in which the hydrocaulus and branches end in non-polyp bearing growing points. The growing points elongate and bud off laterally both polyps and new branches (this method is more typical of thecate hydroid colonies) — (Hyman 1940, p. 405). The picture is obscured somewhat, however, by the hydrorhizal elements of the stem.