Order VII. Seisonaceae.—Marine Rotifers parasitic on the Crustacean Nebalia; males resembling the females. Body elongated, with a slender retractile neck, a much reduced disc, an elongated foot with a terminal perforated disc as in Callidina. Trophi virgate exsertile. Genito-urinary cloaca opening at the base of the neck in the male, at the hinder end of the body in the female. Intestine complete (Seison) or blind (Paraseison).[[275]]
Fam. 24. Seisonidae: Seison Grube; Paraseison Plate; Saccobdella Van Beneden and Hesse.
Habits.—The habitat of Rotifers is well known to the student of pond life. Every dip from a greenish pool will give us a supply, if there be not an excessive contamination by manure; and such pools give us some of the largest and most beautiful forms, such as Hydatina and Brachionus, swimming about among the fibrous Algae and feeding on the organic débris among them. Almost any organic infusions freely exposed to the open air will yield Ploima shortly after the active putrefaction is completed. The finer water-weeds yield most of the beautiful tubicolous forms. A whole group of species and genera are quasi-pelagic in fresh and salt water, constituting a large proportion of the "plankton" or floating life near the surface; and some of these are found in deep water or in the depths of the lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A number of Loricates, such as Notholca and Eretmia, are armed with long spines, which doubtless render floating easier.
Among tubicolous forms Conochilus volvox and Lacinularia racemovata have this pelagic habit, forming floating globular or ovoid colonies, and two species of Floscularia also float freely in their tubes.
The following forms occur in salt or brackish water,[[276]] those marked with an asterisk (*) also occurring in fresh water:—
Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.* Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,* S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides. Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens. Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F. neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C. rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A. aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N. thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus, P. ciliatus. Discobdella nebaliae.
Thus about seventy species are recorded as marine. Synchaeta baltica is truly pelagic, and contributes to the phosphorescence of the ocean.
Other forms again are parasitic. Proales werneckii is found in Vaucheria, a coarse, dark green, thread-like Alga found in fresh water; and the closely allied P. parasita is not uncommon in the beautiful floating green spheres of Volvox.[[277]] Albertia, Drilophagus, and Balatro are parasitic on or in fresh-water Oligochaetes; the curious Seisonaceae are parasitic on Nebalia, a small Crustacean easily obtained in masses of whelk's eggs; the aberrant Bdelloid Discopus attaches itself to the surface of the Holothurian Synapta. Similarly among this last Order Callidina parasitica attaches itself to the limbs of the fresh-water Crustacea Gammarus and Asellus. These are rather commensals than true parasites. The species of Brachionus often attach themselves temporarily to the common water-flea Daphnia.
Besides a few Ploima, the vast majority of the Bdelloids live in or among mosses and their roots. Many Callidina inhabit cup-like hollows in the leaves of the scale mosses (Jungermanniaceae), especially of the genus Frullania. Almost all the members of this Order are susceptible of desiccation and revival; certain species, such as Rotifer vulgaris, Philodina roseola, Adineta vaga, etc., can be readily obtained by moistening gutter dust. The mechanism of the process is as follows: when desiccation is gradual the animals close up their telescopic bodies and excrete gelatinous plugs at either end, which effectually seal them against further drying; if, however, they be dried on a slide without any débris, the process is too rapid for them to protect themselves, and they therefore die. This was dimly seen by others, and clearly demonstrated by H. Davis,[[278]] who records the following experiment:—The Rev. E. J. Holloway, having found Philodina roseola in gutters, placed strips of paper there in the rainy season, and succeeded in obtaining clean gatherings, taking dry groups of a hundred together, having a varnish-like covering all over; and being glued to one another, mostly in one plane, and to the paper, forming a pavement. In the dry condition they resist extremes of temperature; thus Zelinka found Callidina revive after an exposure of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C. (158° F.). They will also resist deprivation of air in a vacuum of an ordinary air-pump, but not the all but perfect exhaustion of the Sprengel pump.
A very curious fact in relation to this Class is that often when a new form is once described from a single locality, fresh and widely distant stations for it rapidly become known.[[279]] Thus Pedalion mirum, first found at Clifton in 1872 by Hudson, was a few years after captured in a small pool above tide-marks on a rocky islet in Torres Straits. Since then it has been recorded from many different European stations, and a second closely allied species has been found in Finland. So a species of Ehrenberg's[[280]] was not seen again till within the last decade or so; but since then it has been independently found and described by six observers, who have given it as many distinct generic names. In the case of Pedalion it may well be that, as Hudson suggests, the species is of southern origin and has followed the flag, the winter egg being conveyed in dust by ships or travellers.