PHORONIS. Phoronis is one of those limbless creatures pop ularly termed "worms." It is, however, so peculiar in its structure that it is given a class to itself, Phoronidea. There are a good many species found all over the world, but they are so alike that they are placed in a single genus Phoronis, although they vary from a in. to 6 in. in length. Phoronis is marine and lives in a leathery tube from which the fore-part of the body can be pro truded. The front part carries a peculiar platform or "lopho phore," crowned with beautiful upright tentacles covered with cilia which are in perpetual motion. In P. hippocrepia the lopho phore is shaped like a horse-shoe and the tentacles are arranged along its edge, those on the concavity of the shoe being somewhat the shorter. All species have lophophores more or less of this shape but the tips of the shoe are in some much more prolonged than in others and even inrolled. The cilia produce a current which sweeps small organisms into the mouth, situated between the outer and inner row of tentacles. It is elongated and slit-like and provided with a covering lip, the epistome, which projects from the concave side of the lophophore. Below the lophophore, on the side of the concavity of the horseshoe, the anus projects as a little papilla. At the sides of the anus two smaller papillae carry the openings of a pair of ciliated tubes which connect the body cavity with the exterior. These tubes (coelomiducts) serve to get rid of the excreta in the body-cavity fluid and to let out the germ cells when these are ripe.
Since the mouth and anus are close together and the animal has a long worm-like body, it is obvious that the alimentary tube will be U-shaped. The long gullet leads into a globular stomach lying at the lower end of the body, from which the long, slender, ascend ing intestine leads to the anus. The first part of the intestine where it leaves the stomach is swollen, lined by ciliated epithelium and devoid of glands. The alimentary tube is enswathed by a true secondary body-cavity or coelom, which is lined by an epithelium the outer layer of which is pressed against the body-wall whilst the inner layer lies against the gut cells. This cavity is divided into
two by a horizontal platform just behind the mouth. The front portion communicates with the cavities of the tentacles and of the epistome. This is the lophophoral coelom : the hinder portion, in which the gullet and intestine lie, and into which the coelomi ducts open, is the trunk-coelom and is traversed by a median septum or mesentery which ties the outer side of the oesophagus to the body wall: in addition, the oesophagus is anchored by two lateral mesenteries. These mesenteries, however, cease before the stomach is reached. The genital cells appear as thickenings of the inner wall of the coelom and both male and female are produced by the same individual in separate swellings, projecting from the wall of the stomach. The trunk coelom is thus divided into three chambers, a right and left lateral on the convex side of the lopho phore and a large rectal chamber on the concave side. In this latter lies the intestine, which is tied to the body wall by its own independent mesentery and is also fused by its side to one of the lateral mesenteries.
Each "coelomiduct" opens into the body-cavity by two funnel shaped openings lined by cilia, a short one communicating with the lateral chamber and a long slit-like one opening into the rectal chamber.
The nervous system is simple, consisting merely of an interlac ing mass of extremely fine fibrils and small nerve-cells which thickens to form a horizontal band passing around the lophophore and giving off a special branch to each tentacle. There is also a thickening in the skin of the epistome.
The life of Phoronis consists merely in expanding the front of the body, erecting the tentacles and working the cilia vigorously; the mouth being opened wide to catch whatever fortune brings. On the approach of danger the whole body is withdrawn within the tube. This is effected by bands of longitudinal muscles in the body-wall—the expansion being brought about by circular muscles, which by compressing the body force fluid into the lophophore and engorge it.