Fig. 157.—Verrucella guadaloupensis, with an epizoic Brittle star (Oph.) of similar colour.
The principal genera are:—Gorgonella, with a ramified flabelliform axis; Ctenocella, with a peculiar double-comb manner of branching; and Juncella, which forms very long unbranched or slightly branched colonies, with club-shaped spicules. All these genera are found in shallow water in the tropical or semi-tropical regions of the world. Verrucella is a genus with delicate anastomosing branches found principally in the shallow tropical waters of the Atlantic shores. Like many of the Gorgonacea, with branches disposed in one plane (flabelliform) Verrucella frequently carries a considerable number of epizoic Brittle stars, which wind their flexible arms round the branches, and thus obtain a firm attachment to their host. There is no reason to suppose that these Brittle stars are in any sense parasitic, as a specimen that bears many such forms shows no sign of injury or degeneration, and it is possible they may even be of service to the Verrucella by preying upon other organisms that might be injurious. An interesting feature of the association is that the Brittle stars are of the same colour as the host, and the knob-like plates on their aboral surface have a close resemblance to the verrucae (Fig. 157).
Order V. Pennatulacea.
The Sea-pens form a very distinct order of the Alcyonaria. They are the only Alcyonarians that are not strictly sedentary in habit, that are capable of independent movement as a whole, and exhibit a bilateral symmetry of the colony. No genera have yet been discovered that can be regarded as connecting links between the Pennatulacea and the other orders of the Alcyonaria. Their position, therefore, is an isolated one, and their relationships obscure.
The peculiarities of the order are due to the great growth and modification in structure of the first formed zooid of the colony. This zooid (Oozooid, Hauptpolyp, or Axial zooid) increases greatly in length, develops very thick fleshy walls, usually loses its tentacles, digestive organs, and frequently its mouth, exhibits profound modification of its system of mesenteries, and in other ways becomes adapted to its function of supporting the whole colony.
Fig. 158.—Diagram of a Sea-pen. L, leaves composed of a row of autozooids; R, rachis; St, stalk; T, anthocodia of the axial zooid, usually suppressed. (After Jungersen.)
The axial zooid shows from an early stage of development a division into two regions: a distal region which produces by gemmation on the body-wall numerous secondary zooids, and becomes the rachis of the colony; and a proximal region which becomes the stalk or peduncle, and does not produce buds (Fig. 158). The secondary zooids are of two kinds: the autozooids and the siphonozooids. The former have the ordinary characters of an Alcyonarian zooid, and produce sexual cells; the latter have no tentacles, a reduced mesenteric system, and a stomodaeum provided with a very wide siphonoglyph.
The arrangement of the autozooids and siphonozooids upon the axial zooid is subject to great modifications, and affords the principal character for the classification of the order. In the Pennatuleae the autozooids are arranged in two bilaterally disposed rows on the rachis, forming the leaves or pinnae of the colony (Fig. 158). The number in each leaf increases during the growth of the colony by the addition of new zooids in regular succession from the dorsal to the ventral side of the rachis[[385]] (Fig. 159). In other Pennatulacea the autozooids are arranged in rows which do not unite to form leaves (Funiculina), in a tuft at the extremity of a long peduncle (Umbellula), scattered on the dorsal side of the rachis (Renilla, Fig. 160), or scattered on all sides of the rachis (Cavernularia, Fig. 161). In those forms in which the autozooids are scattered the bilateral symmetry of the colony as a whole becomes obscured. The siphonozooids may be found on the leaves (Pteroeides), but more frequently between the leaves or rows of autozooids, or scattered irregularly among the autozooids. Usually the siphonozooids are of one kind only, but in Pennatula murrayi there is one specially modified siphonozooid at the base of each leaf,[[386]] which appears to have some special but unknown function.