Of this class are the curious _Pennatulidæ_, one of which is commonly known as the Sea-Pen. The three species of this class of _Polyps_ known to inhabit the British seas, are so distinct from each other that they form at the same time three distinct genera. The most beautiful of the three is the _Pennatula phosphorea_, the Sea-Pen, which is not uncommon on some parts of our northern coasts. It is represented in Plate VII. The purple branches, or pinnæ, of the upper portion, form the feathered part of the quill pen to which it is likened; the bare portion of the polypidom below having certainly some resemblance to the quill. This curious zoöphytic form is often seen in an erect position, planted, as it were, in the mud like a miniature purple Pine, though it is capable of motion through the water from place to place, by some action of its organs which has not been accurately detected. It is one of the handsomest of our British Zoöphytes. The polypidom, or trunk, is three or four inches long, fleshy, and of a purplish red. It is naked at the lower end, and feathered above with long, closely-set pinnæ, along the margins of which the polyp-cells are placed. The pinnæ are curved backward, and capable of either separate or united motion. They are supposed by some to be capable of the action of regular oars; but this is very doubtful, though their bearing on the polypidom, which is strengthened by an internal column of calcareous or bony matter, would give them considerable power for that purpose. The creature’s specific name, _phosphorea_, must not lead to the supposition that it always emits a phosphorescent light, for it is only when irritated that this is produced. If plunged into fresh-water, it scatters a shower of phosphoric sparks in all directions, which forms a magnificent and curious spectacle, far more brilliant, no doubt, than the fabled hues of the dying dolphin. The _Virgularia mirabilis_ is another of this class of creatures, almost as elegant as the Sea-Pen, but more slender, and in the form of a branching rod, as its name imports. (See Plate VIII.) The Sponges form a curious class of Zoöphytes, which have perhaps a much closer affinity to plants than any other. They are occasionally very sportive, and curious in their forms; and Dr. Johnston enumerates fifty-six species belonging to our coasts; they are, however, unsuited to the Aquarium in the present state of our knowledge; and when portions of rock are collected on the shore, for the tank, care should be taken to clear off any Sponge formations that are perceived, as their certain and rapid decay would be liable to injure the condition of the whole colony of the Aquarium. [Illustration] ------------------------------------------------------------------------ CHAPTER VI. THE MOLLUSCS, ETC.