Several fossil forms have been described. Peridinium is certainly found fossil in the firestone of Delitzet, belonging to the Cretaceous. A full monograph of the group under the name "Peridiniales" was published by Schütt.[[144]]
Fig. 47.—Pyrocystis fusiformis, Murray. × 100. From the surface in the Guinea Current. (From Wyville Thomson.)
The Cystoflagellates contain only two genera,[[145]] Noctiluca, common at the surface of tranquil seas, to which, as its name implies, it gives phosphorescence, and Leptodiscus, found by R. Hertwig in the Mediterranean. Noctiluca is enormous for a Flagellate, for with the form of a miniature melon it measures about 1 mm. (1⁄25") or more in diameter. In the depression is the "oral cleft," from one end of which rises, by a broad base, a large coarse flagellum, as long as the body or longer and transversely striated. In front of the base of the flagellum are two lip-like prominences, of which one, a little firmer than the other, and transversely ridged, is called the tooth; at the junction of the two is a second, minute, flagellum, usually called the cilium. Behind these the oral groove has an oval space, the proper mouth; behind this, again, the oral groove is continued for some way, with a distinct rod-like ridge in its furrow. The whole body, including the big flagellum, is coated by a strong cuticular pellicle, except at the oblong mouth, and the lips and rod are mere thickenings of this. The cytoplasm has a reticulate arrangement: the mouth opens into a central aggregate, from which strands diverge branching as they recede to the periphery, where they pass into a continuous lining for the cuticular wall, liquid filling the interspaces. The whole arrangement is not unlike that found in many plant-cells, but the only other Protists in which it occurs are the Ciliata Trachelius (Fig. 56, p. [153]) and Loxodes. The central mass contains the large nucleus. Noctiluca is an animal feeder, and expels its excreta through the mouth. The large flagellum is remarkable for the transverse striation of its plasma, especially on the ventral side. The cuticle may be moulted as in the Dinoflagellates. As a prelude to fission the external differentiations disappear, the nucleus divides in the plane of the oral groove, and a meridional constriction parts the two halves, the new external organs being regenerated. Conjugation occurs also, the two organisms fusing by their oral region; the locomotive organs and pharynx disappear; the conjoined cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or fertilisation nucleus. This conjugation is followed by sporulation or brood-formation.[[146]]
Fig. 48.—Noctiluca miliaris, a marine Cystoflagellate. (From Verworn.)
The nucleus passes towards the surface, undergoes successive fissions, and as division goes on the numerous daughter-nuclei occupy little prominences formed by the upgrowth of the cytoplasm of the upper pole. The rest of the cytoplasm atrophies, and the hillocks formed by the plasmic outgrowths around the final daughter-nuclei become separate as so many zoospores (usually 256 or 512); each of these is oblong with a dorsal cap-like swelling, from the edge of which arises a flagellum pointing backwards; parallel to this the cap is prolonged on one side into a style also extending beyond the opposite pole of the animal.[[147]] In this state the zoospore is, to all outward view, a naked Dinoflagellate, whence it seems that the Cystoflagellates are to be regarded as closely allied to that group. Leptodiscus is concavo-convex, circular, with the mouth central on the convex face, 1-flagellate, and attains the enormous size of 1.5 mm. (1⁄16") in diameter.
The remarkable phosphorescence of Noctiluca is not constant. It glows with a bluish or greenish light on any agitation, but rarely when undisturbed. A persistent stimulus causes a continuous, but weak, light. This light is so weak that several teaspoonsful of the organism, collected on a filter and spread out, barely enable one to read the figures on a watch a foot away. As in other marine phosphorescence, no rise of temperature can be detected. The luminosity resides in minute points, mostly crowded in the central mass, but scattered all through the cytoplasm. A slight irritation only produces luminosity at the point touched, a strong one causes the whole to flash. Any form of irritation, whether of heat, touch, or agitation, electricity or magnetism, is stated to induce the glow. By day, it is said, Noctiluca, when present in abundance, may give the sea the appearance of tomato soup.
The earliest account of Noctiluca will be read with interest. Henry Baker writes in Employment for the Microscope:[[148]]—"A curious Enquirer into Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own Observations that the Sparkling of Sea Water is occasioned by Insects. His Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of Sea Water I send with this are some of the Animalcules which cause the Sparkling Light in Sea Water; they may be seen by holding the Phial up against the Light, resembling very small Bladders or Air Bubbles, and are in all Places of it from Top to Bottom, but mostly towards the Top, where they assemble when the Water has stood still some Time, unless they have been killed by keeping them too long in the Phial. Placing one of these Animalcules before a good Microscope, an exceeding minute Worm may be discovered, hanging with its Tail fixed to an opake Spot in a Kind of Bladder, which it has certainly a Power of contracting or distending, and thereby of being suspended at the Surface, or at any Depth it pleases in the including Water.'"
"The above-mentioned Phial of Sea Water came safe, and some of the Animalcules were discovered in it, but they did not emit any Light, as my Friend says they do, upon the least Motion of the Phial when the Water is newly taken up. He likewise adds, that at certain Times, if a Stone be thrown into the Sea, near the Shore, the Water will become luminous as far as the Motion reacheth: this chiefly happens when the Sea hath been greatly agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's correspondent, took for a worm was the large flagellum.