{341}It is not known for certain when the fertilisation of the ova is effected, but in _Alcyonium digitatum_, and in the majority of the Alcyonarians, it probably takes place after the discharge of the ova from the zooids. A few forms are, however, certainly viviparous, the larvae of _Gorgonia capensis_ being retained within the coelenteric cavity of the parent zooid until they have grown to a considerable size. The other viviparous Alcyonarians are _Corallium nobile_ (de Lacaze Duthiers), the "Clavulaires petricoles," and _Sympodium coralloides_ (Marion and Kowalevsky), and three species of _Nephthya_ found at depths of 269 to 761 fathoms (Koren and Danielssen). The general features of the development are very similar in all Alcyonarians that have been investigated. The egg contains a considerable amount of yolk, and undergoes a modified form of segmentation. The free-swimming larva is called a "sterrula." It consists of an outer layer of clear ciliated ectoderm cells, surrounding a solid endodermic plasmodium containing the yolk. As the yolk is consumed a cavity appears in the endoderm, and the larva is then called a "planula" (Fig. 150). The mouth is subsequently formed by an invagination of the ectoderm at the anterior pole. The development of the mesenteries has not yet been fully described. [Illustration: FIG. 150.—Ciliated "planula" larva of _Alcyonium digitatum_. _Ec_, Ectoderm; _End_, endoderm.] CLASSIFICATION.—The sub-class Alcyonaria may conveniently be classified as follows:— Grade A. PROTALCYONACEA. Grade B. SYNALCYONACEA. Order 1. STOLONIFERA. Order 2. COENOTHECALIA. Order 3. ALCYONACEA. Order 4. GORGONACEA. Order 5. PENNATULACEA. {342}GRADE A. PROTOALCYONACEA. This Grade includes those genera which, like many sea-anemones, do not reproduce by continuous gemmation to form colonies. Several genera have been described, and they have been placed together in one family called the HAIMEIDAE. _Haimea funebris_, M. Edwards, was found off the coast of Algeria; _H. hyalina_, Koren and Danielssen, in Norway; _Hartea elegans_, Wright, from the Irish coast; _Monoxenia darwinii_, Haeckel, from the Red Sea, and a large new species found by the "Siboga" Expedition in deep water off Ceram. All these species, however, are very rare, and there is no satisfactory evidence at present that they remain solitary throughout life. GRADE B. SYNALCYONACEA. The sub-division of the Synalcyonacea into orders presents many difficulties, and several different classifications have been proposed. Only two orders of the five that are here recognised are clearly defined, namely, the Coenothecalia, containing the single living genus _Heliopora_, and the Pennatulacea or Sea-pens; the others are connected by so many genera of intermediate characters that the determination of their limits is a matter of no little difficulty. ORDER I. STOLONIFERA.