The length of the individuals varied from three to six inches, and their diameter, which is not very uniform, averaged one-eighth of an inch. At one end, which, since it bears the mouth, we may call the oral end, is the very characteristic tentacular crown surrounding the mouth on all sides but one, where there is a slight break in its continuity. The crown of tentacles or lophophore is flattened, and the two ends drawn out, and each is coiled into a spiral (Fig. 229); between the bases of these two spirals three ridges can be seen, each ending in a pore; the median opening is the anus, the two lateral are the openings of the nephridia or kidneys, which also serve as ducts for the reproductive organs. The anus is thus approximated to the mouth, and since the continuity of the tentacular crown is broken at a spot just between the two, there would be nothing to separate these orifices if it were not for the presence of the epistome, a projection or flap of the body-wall which overhangs the mouth between it and part of the crown of tentacles.
The extent to which the ridge bearing the tentacles is incurved at each side varies in different species. In Ph. kowalevskii and Ph. psammophila the ends are only slightly turned in, so that the crown of tentacles is truly horse-shoe shaped; but in Ph. australis they are turned in and form three coils on each side. The number of tentacles also varies, being very numerous in Ph. australis and Ph. buskii—the latter having as many as 300, whilst the other species as a rule have from 60 to 90. The bases of the tentacles are fused for a short distance with one another, forming a thin membrane.
Fig. 228.—A specimen of Ph. buskii M‘Int. removed from its tube and seen from behind, × about 2. (After M‘Intosh.)
The rest of each tentacle is free, and its inner surface, or that turned towards the mouth, is covered with long cilia, which, by the currents they set up, doubtless serve to bring food to the mouth. The tentacles are hollow, and their cavity is kept open by a stiffening of the tissue, which almost resembles an internal skeleton; the cavity communicates with the anterior part of the general body-cavity, and up it runs a single blood-vessel containing red blood. A single nerve is also distributed to each tentacle.
At the base of the two spirals of the tentacular crown lie two ciliated pits, regarded by Caldwell and M‘Intosh[[495]] as sensory organs, but Benham looks upon them as glandular in structure and function. Perhaps they secrete the substance from which the tubes are formed.
The skin is covered by a delicate cuticle secreted by the underlying epidermis; within the latter is a well-marked basement membrane, and beneath this a layer of circular muscle fibres; these surround a layer of longitudinally-arranged fibres, which do not form a continuous sheet but are arranged in bundles. In both layers the fibres are unstriated. The longitudinal fibres are covered on their inner side by a layer of flat pavement cells, which line the general cavity of the body.
This space, the body-cavity, is divided into two parts by the presence of a diaphragm or septum which runs across from one side of the body to the other about the level of the ridge bearing the tentacular crown. The anterior space is continuous with the cavities of the tentacles and of the epistome. The partition is pierced by the blood-vessels and the oesophagus, but the rest of the alimentary canal, including the anus, the kidneys, and the reproductive organs, all lie in the posterior half of the body-cavity behind the diaphragm. This portion of the body-cavity is further subdivided by the presence of three longitudinal mesenteries supporting the alimentary canal and running between it and the body-wall. One of these mesenteries runs along the outside of the alimentary canal throughout its whole length, attaching both the descending and ascending limbs of the U-shaped tube to the body-wall. The other two are lateral mesenteries, which pass from the body-wall to the sides of the oesophagus. These mesenteries therefore divide the body-cavity into three spaces—one in which the rectum lies, which may be called the rectal, and two lateral; owing to the fact that the lateral mesenteries end before they reach the bend of the alimentary canal, the three chambers are in free communication one with another. The body-cavity is further traversed by irregular strands of tissue which run from the body-wall to the various organs. It contains a corpusculated fluid.
The alimentary canal (Fig. 230) consists of a U-shaped tube which may be divided into four regions. The mouth (m) leads into the oesophagus (oe), which gradually enlarges into the stomach (st) situated just before the bend; a constriction just at the bend separates the stomach from the intestine (int), and this leads into the rectum (r), which terminates in the anus (an). The first three divisions of the alimentary canal are ciliated, but the rectum is not; the walls of the stomach also contain glandular cells, but there are no special glands opening into any part of the tract.
