Order IV. Zoanthidea.
This order of Zoantharia consists of a number of solitary or colonial Anemones that do not form a skeleton of horn or carbonate of lime, and are distinguished from the Actiniaria by the peculiar arrangement of their mesenteries.
Fam. 1. Zoanthidae.—Sphenopus is a solitary coral and terminates aborally in a small sucker-like base, by which it may be attached to foreign bodies. The genera Gemmaria and Isaurus include solitary forms.
In the majority of the species of Zoanthids, however, a basal encrusting stolon is formed, which may be thick and fleshy or membranous, or may consist of a plexus of bands from which several zooids rise and on which the new buds are formed.
The tentacles are numerous, simple, usually short, and arranged in one or two circles on the margin of the disc. Most Zoanthidae are encrusted with sand, shell fragments, or sponge spicules, but Zoanthus and Isaurus are naked. The foreign particles that form the incrustation are firmly attached to the ectoderm, and as a rule many of them sink down into the mesogloea to give additional support to the body-wall. It is the presence of so much incorporated sand that frequently gives these Zoantharia such a very brittle character. The stomodaeum usually exhibits a well-marked ventral siphonoglyph. The mesenteries consist of a pair of complete ventral directives, a pair of incomplete dorsal directives, while of the remaining protocnemes the lateral mesenteries which are first and second in the order of appearance are complete, the sixth is incomplete, whereas the fifth is complete in the Macrocneminae and incomplete in the Brachycneminae. Duerden[[421]] has found in specimens of three species that the arrangement of the mesenteries is "brachycnemic" (the sixth protocneme imperfect) on one side and "macrocnemic" (the sixth protocneme perfect) on the other. The metacnemes appear in the spaces between the sixth protocnemes and the ventral directives in unilateral pairs, of which one becomes complete and the other always remains incomplete (Fig. 163, 4, p. [368]).
The Zoanthidae are usually dioecious, but hermaphroditism undoubtedly occurs in the genera Zoanthus and Isaurus. Little is known of their development, but a larval form discovered by Semper off the Cape of Good Hope, of cylindrical shape, with an opening at each end and distinguished by a longitudinal band of cilia running from one end to the other, is probably the larva of a Zoanthid. It is commonly known as Semper's larva. Other larvae provided with a ring of cilia have also been attributed to this group.
A great many Zoanthidae are epizoic in habit. Thus several species of Epizoanthus form colonies on the shells of Gasteropods inhabited by hermit crabs. Parazoanthus tunicans is found on the stem of a Plumularia; Parazoanthus separatus, from Jamaica, is associated with a sponge. The base of the bundle of long spicules of the Sponge Hyalonema (p. [204]) is almost invariably sheathed by a colony of Epizoanthus stellaris.
The only genera occurring within the British area are Epizoanthus (with six species), Parazoanthus (with four species), and Zoanthus sulcatus.
Of the species of Epizoanthus, E. incrustatus is fairly common, in depths of twenty to eighty fathoms on all our coasts, and is frequently commensal with different species of hermit crabs, while E. paguriphilus is found in much deeper water off the west coast of Ireland and is always commensal with hermit crabs. Parazoanthus anguicomus is found at depths of a hundred fathoms off the Shetlands and west of Ireland, and is usually associated with various species of Sponges.
Gerardia savalia is the largest "black coral" of the Mediterranean. The colony begins by encrusting the stem of one of the Gorgoniidae, but soon surpassing its support in growth, it forms a basal horny skeleton of its own and builds up very large branching colonies. A specimen in the British Museum,[[422]] from twenty fathoms off the island Negropont, is two metres high and two metres wide. The genus appears to be related anatomically to Parazoanthus.