The Pennatulidae, or "Sea-pens," are defined by their free habit, and by the possession of a sclerobasic, rod-like corallum, sometimes associated with sclerodermic spicules.

Pennatula, or the "Cock's-comb," consists of a free coeno-sarc, the upper end of which is fringed on both sides with feather-like lateral pinnae, which bear the polypes; whilst its proximal end is smooth and fleshy, and is probably sunk in the mud of the sea-bottom. This latter portion of the coeno-sarc is likewise strengthened by a long, slender, styliform sclerobasis, resembling a rod in shape, whilst spicula occur also in the tentacles and ectoderm. The general colour of Pennatula is a deep reddish purple, the proximal extremity of the coenosarc being orange-yellow. The common British species (Pennatula phosphorea) varies from two to four inches in length, and is found on muddy bottoms in tolerably deep water. Its specific name is derived from the fact that it phosphoresces brilliantly when irritated.

In Virgularia (fig. 82), which, like Pennatula, occurs not uncommonly in British seas, the actinosoma is much longer and more slender than in the preceding, and the polype-bearing fringes are short. The polypes have eight tentacles. The sclerobasis is in the form of a long calcareous rod, like a knitting-needle, and part of it is usually naked. No spicula are found in the tissues of Virgularia. In the nearly-allied Pavon-aria the polype-mass is quadrangular in shape.

Fig. 81.   Colony of Veretillum cyno morium, of the natural size, with the polypes protruded.

Fig. 81. - Colony of Veretillum cyno-morium, of the natural size, with the polypes protruded.

Fig. 82.   Pennatulidae. Virgularia mirabilis. a A portion of the stem in the living condition, enlarged; b Portion of the stem in its dead condition.

Fig. 82. - Pennatulidae. Virgularia mirabilis. a A portion of the stem in the living condition, enlarged; b Portion of the stem in its dead condition.

In Veretillum (fig. 81), the upper portion of the colony is short and club-shaped, and carries the polypes all round its circumference, and the same is the case in Cophobelemnon; whilst in Renilla the polypes are unilateral, and the polyp-iferous coenosarc is thin and reniform.

In many of the Pennatulidae, as originally shown by Kolliker, the actinosoma consists of two classes of zooids - the one composed of sexually mature polypes, the other, more numerous, of sexless polypes - which have a body-cavity and stomach, but have neither mouth nor tentacles. These sexless zooids may be distributed promiscuously over the whole actinosoma ( Veretillum, &c), or they may be restricted to definite regions (Pennatula, Virgularia). Whilst many of the Pennatulidae seem to live habitually sunk partially in the mud of the sea-bottom, others are found freely floating in the water, and their mode of life is not completely understood.