Non-colonial, free-swimming and pelagic Coelenterata, globular, cylindrical, rarely band-like in shape. The month is at one pole of the principal axis, a sensory organ and otolithic mass with two excretory apertures at the other. Eight rows of ciliary or ctenophoralplates radiate meridionally from the sensory organ. There is, as a rule, a pair of tentacles, retractile into pouches, and provided with adhesive cells. Hermaphrodite. Exclusively marine.
An axis passing through the otolithic mass and the mouth is the principal axis of the body. It has two poles, an oral and aboral. Two vertical planes at right angles to each other pass through this axis. One is parallel to the longer diameter of the stomach, and may be termed the stomachal or lateral plane: the other is parallel to the longer diameter of the funnel, and may be termed the funnel plane, median or sagittal plane. The funnel plane divides the body into a left and right half, the stomachal into an anterior or dorsal, and a posterior or ventral half. Each half consists of two quadrants1. Two axes, a stomachal or lateral and a funnel or dorso-ventral axis, pass respectively through the stomachal and funnel planes. A horizontal plane at right angles to the planes of the principal axis may be termed the equatorial or dorso-ventral plane. The two tentacles lie one at each end of the funnel plane. The eight meridional vessels and cteno-phoral rows are distributed four in each lateral half, or two to each quadrant of the body.
The row of plates or the vessel in each quadrant which adjoins the funnel plane or tentacle, is termed 'subtentacular'; that which adjoins the stomachal plane 'subventral.'
1How far weight can be laid on this negative character is doubtful. Thread-cells are found in Infusoria among Protozoa; in Aeolidia (Gastropoda); and structures which are generally compared with them in Turbellaria. The late Professor Balfour thought it possible that the layers, i. e. 'ectoderm, endoderm and mesoblast did not correspond with the similarly named layers in the Coelenterata and other Metazoa.' He was influenced in his views by the peculiar character of the amphiblastula larva and by current opinions on the function of the collared cells - that they were respiratory, not digestive. There are, however, different forms of Sponge larvae; and authorities are even as yet by no means agreed as to the function of the collared cells.
The typical shape of a Ctenophore as seen in a Pleurobrachia is ovate, the long axis of the oval coinciding with the principal axis. The mouth leads into a flattened ectodermic stomach, the plane of the flat sides coinciding with the stomachal plane. Towards the aboral aspect, the stomach leads in turn into the endodermic funnel, the longest diameter of which coincides with the funnel plane. The funnel gives origin to two 'per-radial' vessels, which run more or less horizontally outwards in the funnel plane in opposite directions. At their outer ends these vessels bifurcate. And the four vessels thus formed belong one to each of the four quadrants and are known as 'interradial' vessels from the direction they take. A vessel to each tentacle springs from the angle or fork between each pair of interradials. The four interradial vessels bifurcate in turn giving origin to adradial vessels, eight in number, two to each quadrant. The adradials open one into each of the eight meridional or ctenophoral vessels which end blindly at their oral and aboral ends. The eight rows of ctenophoral plates coincide with the eight meridional vessels. A vessel originates from each of the perradial vessels close to its origin, runs in an oral direction close to the flat side of the stomach, and ends blindly.
These two vessels are the 'paragastric canals.'
At the centre of the aboral pole is a pyramidal depression surmounted by a hyaline bell. The depression is elliptical or roughly hexagonal in outline, the longer axis of the ellipse or hexagon coinciding with the stomachal plane. The space inclosed is the central nervous system or sensory area. The ectoderm cells of this area are ciliated. Certain of them disposed in a circular row are provided with very long cilia which are bent like the figure 2, and are fused in groups to form four triangular cilia-plates. The tips of these plates give support to a spherical mass of agglutinated otoliths which consist of Calcium phosphate. Rows of similar but small plates, the four nerves or ciliated furrows, diverge outwards and interradially from the bases of the four cilia plates in question. They pass through four openings in the bell, divide each into two, and the eight nerves thus formed abut one against the first ctenophoral plate of each of the eight rows. The long axis of the sensory ellipse is prolonged outwards on each side, and in the same plane by the polar plates which in the adult are rectangular in outline. They are ciliated and their margins thickened.
The central area of each polar plate leads into the bell by an aperture; its cilia work towards this aperture and renew the water within the bell. The central nervous system can be retracted by special muscles.
1The 'stomachal' plane of Chun corresponds to the 'coeliac' plane of Agassiz, the 'sagittal' plane of Claus, Haeckel, and Hertwig, and the 'transverse' plane of Fol. Chun's 'funnel' plane is the 'diacoeliac' plane of Agassiz, the 'transverse' of Claus and Hertwig, the 'lateral' of Haeckel, and the 'tentacular' of Fol.
The funnel extends upwards beneath the central nervous system as the funnel vessel; it bifurcates and each branch forks again into two ampullae, making four in all, one to each interradius. Two, which correspond to the anterior left interradius and the posterior right, remain blind, whilst the other two are prolonged each into an excretory or anal tube opening externally in the right anterior interradius, and the posterior left just outside the polar areae. These apertures are usually closed but are opened by the animal for the escape of water and faecal particles1.