Fam. 2. Callianiridae.—Two or four wing-like processes, into which the longitudinal canals extend, are found at the aboral pole. Callianira has two of these processes arranged in the transverse plane, and Lophoctenia has four. Callianira is found in the Mediterranean and in the Atlantic from the Arctic to the Antarctic waters.
Fam. 3. Pleurobrachiidae.—The body is almost spherical in form, and the eight ribs are equal in length.
This family includes the genus Pleurobrachia, in which the ribs extend for a considerable distance along the lines of longitude of the spherical body, but do not reach either the oral or the aboral areas. P. pileus is the commonest British Ctenophore, and may be found in shoals in May, June, and July at the surface of the sea or cast up on the sand as the tide ebbs. It is widely distributed in the North Atlantic waters. P. rhodopis of the Mediterranean has rather shorter ribs than P. pileus. Two new species have recently been described from the Malay Archipelago.[[430]] Hormiphora (Fig. 180, p. [413]) differs from Pleurobrachia in having much shorter ribs, and in possessing two kinds of pinnae on the tentacles, those of the ordinary kind and others much larger and sometimes palmate in character. This genus has a world-wide distribution.
In Lampetia and Euplokamis the body is more cylindrical in shape than it is in the other genera, but the ribs and subjacent longitudinal canals extend up to the margin of the aboral field. Both these genera occur in the Mediterranean, but Lampetia is also found in the Malay Archipelago.
Order II. Lobata.
The body is considerably flattened in the transverse plane, and the sagittal areas are extended into the form of two wide peristomial lobes. The oral ends of the areas between the transverse and sagittal ribs are extended to form four flaps, called the "auricles." There are no tentacles nor tentacle-sheaths of the ordinary kind in the adult form; but numerous tentilla, similar in some respects to the pinnae of the tentacles of other Ctenophora, form a fringe round the margin of the auricles and the peristome. A single pair of long, filamentous, non-retractile tentacles arise from the sides of the peristomium in Eucharis multicornis. These tentacles have no sheaths, and do not bear pinnae. They are probably not homologous with those of other Ctenophora.
The characters that separate the families of Lobata are chiefly those of varying size, shape, and position of the peristomial lobes and auricles. In the Lesueuriidae the peristomial lobes are rudimentary; in the other families they are moderately or very large. In the Bolinidae the auricles are short, but in most of the other families they are long and ribbon-like. In Eucharis they can be spirally twisted in repose.
The modifications of the external form seen in the Lobata are accompanied by some modifications of the internal structure. Among these, perhaps the most interesting is a communication between the transverse longitudinal and the paragastric canals, and the long convoluted tubes given off to the peristomial lobes by the sagittal longitudinal canals. Very little is known about the life-history and development of most of the Lobata, but Chun has shown that in Eucharis and Bolina there is a Cydippiform larval stage which produces ripe ova and spermatozoa. This is followed by a period of sterility, but when the adult characters are developed they become again sexually mature. To this series of sexual phenomena the name "Dissogony" is given.
Fig. 181.—Ocyroe crystallina. Ab, aboral sense-organ; au, auricle; Can, diverticulum from the paragastric canal passing into peristomial lobe; Ct, costae; M, mouth; Par, paragastric canal passing outwards to join one of the transverse subcostal canals; P.L, peristomial lobe; w, wart-like tubercles on the lobe. (After Mayer.)