In the genus _Pennatula_, the polyps are disposed in transverse rows upon the outer and inner edge, in a series of prolongations in the form of a feather. These winged species of polypidom are somewhat scythe-shaped, well developed, and furnished with a great quantity of pointed spiculæ, which are constituted of bundles at the base of the calyx. The space between the two rows of appendages is sometimes a tissue, sometimes scaly, sometimes granulous. Of the _Pennatula_ five species are known, and all of them appear to be gifted with phosphorescent properties. We may note among these species _Pennatula spinosa_ (Fig. 61), which inhabits the Mediterranean, and takes its name from its colour; _Pennatula phosphorea_, which abound in most European seas, being found in great plenty, clinging to the fishermen's lines round our own northern shores, more especially when they are baited with mussels. _P. phosphorea_ is of a reddish purple, the base of the smooth stalk pale; the raches roughened with close-set papillæ, and furrowed down the middle; pinnæ close; polyp cilia uniserial, tubular, with spinous apertures. (Sibbald.) Bohadsch says the _Pennatulæ_ swim by means of their pinnæ, which they use as fishes do their fins. Ellis says, "It is an animal that swims about in the sea, many of them having a muscular motion as they swim along;" these motions being effected, as he tells us in another place, by means of the pinnules or feather-like fins, "evidently designed by Nature to move the animal backward or forward in the sea." Cuvier tells us they have the power of moving by the contraction of the fleshy part of the polypidom, and also by the combined action of its polyps. Dr. Grant says, "A more singular and beautiful spectacle could scarcely be conceived than that of a deep purple _P. phosphorea_ with all its delicate transparent polypi expanded, and emitting their usual brilliant phosphorescent light, sailing through the still and dark abyss, by the regular and synchronous pulsations of the minute fringed arms of the whole polypi;" while Linnæus tells us that "the phosphorescent sea-pens which cover the bottom of the ocean cast so strong a light, that it is easy to count the fishes and worms of various kinds which sport among them." [Illustration: Fig. 61. Sea-pen, Pennatula spinosa. (Edes.)]