[Illustration: Fig. 18. Single individual of Astrangia, fully expanded.]
[Illustration: Fig. 19. Magnified lasso-cell of Astrangia.]
The description of the internal arrangement of parts in the Actinia applies in every particular to these corals, with the exception of the hard deposit in the lower part of the body. As in all the Polyps, radiating partitions divide the main cavity of the body into distinct separate chambers, and the tentacles increasing by multiples of six, numbering six in the first set, six in the second, and twelve in the third, are hollow, and open into the chambers. But the feature which distinguishes them from the soft Actiniæ, and unites them with the corals, requires a somewhat more accurate description. In each individual, a hard deposit is formed (Fig. 20), beginning at the base of every chamber, and rising from its floor to about one fifth the height of the animal at its greatest extension. This lime deposit does not, however, fill the chamber for its whole width, but rises as a thin wall in its centre. (See Figs. 13, 17.) Thus between all the soft partitions, in the middle of the chambers which separate them, low limestone walls are gradually built up, uniting in a solid column in the centre. These walls run parallel with the soft partitions, although they do not rise to the same height, and they form the radiating lines like stiff lamellæ, so conspicuous when all the soft parts of the body are drawn in. The mouth of the Astrangia is oval, and the partitions spread in a fan-shaped way, being somewhat shorter at one side of the animal than on the other. The partitions extend beyond the solid wall which unites them at the periphery, in consequence of which, this wall is marked by faint vertical ribs.
[Illustration: Fig. 20. Limestone parts of an individual of Astrangia; magnified.]
[Illustration: Fig. 21. Single individual of Halcyonium seen from above; magnified.]
We come now to the Halcyonoids, represented in our waters by the Halcyonium (Fig. 22). In the Halcyonoids, the highest group of Polyps, the tentacles reach their greatest limitation, which, as above mentioned, is found to be a mark of superiority, and, connected with other structural features, places them at the head of their class. The number of tentacles throughout this group is always eight. They are very complicated (Fig. 21), in comparison with the tentacles of the lower orders, being deeply lobed, and fringed around the margin. Our Halcyonium communities (Fig. 22) usually live in deep water, attached to dead shells, though they may occasionally be found growing at low-water mark, but this is very rare. They have received a rather lugubrious name from the fishermen, who call them "dead-men's fingers," and indeed, when the animals are contracted, such a community, with its short branches attached to the main stock, looks not unlike the stump of a hand, with short, fat fingers. In such a condition they are very ugly, the whole mass being somewhat gelatinous in texture, and a dull, yellowish pink in color. But when the animals, which are capable of great extension, are fully spread, as in Fig. 22, such a polyp-stock has a mossy, tufted look, and is by no means an unsightly object. When the individuals are entirely expanded, as in Fig. 23, they become quite transparent, and their internal structure can readily be seen through the walls of the body; we can then easily distinguish the digestive cavity, supported for its whole length by the eight radiating partitions, as well as the great size of the main digestive cavity surrounding it. Notwithstanding the remarkable power of contraction and dilatation in the animals themselves, the tentacles are but slightly contractile. This kind of community increases altogether by budding, the individual polyps remaining more or less united, the tissues of the individuals becoming thicker by the deposition of lime nodules, and thus forming a massive semi-cartilaginous pulp, uniting the whole community. In the neighborhood of Provincetown they are very plentiful, and are found all along the shores of our Bay in deep water.