Some Dinoflagellata have been observed encysted in a resting condition. Examples naked or protected by a fine cuticle are occasionally met with, probably derived from such cysts. Growth, however, appears to be accompanied by moults which may also be produced by unfavourable conditions. Some marine species, e. g. Ceratium tripos, C. fusus, Proro-centrum micans, are phosphorescent. Fossil species of Peridinium have been recognised in rocks belonging to the cretaceous series in Germany.

1The two vacuoles usually flow together, and at the same time a fine canal is stated to be visible leading to the surface, towards the bases of the flagella. The view that the vacuoles serve to take in fluid nourishment, or are analogous to the sap-vacuoles of vegetable cells, is improbable.

2Bergh, e. g. observed solid masses of apparently ingested food in Gymnodinium gracile, G. spirale, and Polykrikos auricularia. An oesophageal tube is said to exist in Prorocentrum, and by Entz in Gymnodinium pulvisculus (Z. W. Z. xxxviii. p. 188). Kent observed his G. marinum chasing monads, and swallowing them by a mouth near the base of the flagellum (Manual of Infusoria, i. p. 444), and Maupas observed an undetermined marine Peridinium devouring large Infusoria by means of a sucking tentacle (A. Z. Expt. ix. p. 365). >

3Pyriform fixed cysts have been observed in Exuviaella lima; so too in Gymnodinium pulvis-culus attached to the tail of an Appendicularia; Pouchet, J. de l'Anat. et Physiol. (Robin), xxi. p. 61 et seqq.

(iv.) Cystofiagellata s. Rhynchoflagellata. Two genera, both marine, are contained in this sub-class: - Noctilnca with probably only one species, N. miliaris, which is widely distributed, and Leptodiscus medusoides, found by R. Hertwig at Messina. In both genera the protoplasm is broken up into a central mass with branching cords extending from it.

Noctiluca is from one point of view globular, from another at right angles to the first, somewhat kidney-shaped. It is divisible into two halves, a right and left. In the median plane is an elongated depression, the peristome, at the base of which is a slit-like mouth. At one end, the anterior, of the peristome is a flat tentacle, and close behind it on the right side two small ridges one behind the other, one ending in three or four points, the tooth, the other somewhat semicircular in outline, the lip. A flagellum is inserted at the anterior end of the lip. Each margin of the posterior end of the peristome is bordered by a slight fold; the two folds approach one another and are continued onwards behind the peristome as the 'staff-organ,' until they meet at a point corresponding to one end of a diameter, which joins at its other end the anterior border of the peristome. In size the animal may attain .5-1 mm. (1/25 in.) or even more (Giglioli).

The surface of the body is covered by a delicate clear layer, the cuticle of some authors, which is probably a superficial stratum of the protoplasm differing somewhat from the rest in physical and chemical characters. A mass of protoplasm containing the nucleus lies beneath the peristome, the mouth opening directly into it. From this mass radiate branching and anastomosing cords, their fine ends ending in a thin superficial layer. The cords are separated by large vacuoles; they contain, as does also the superficial layer, small non-contractile vacuoles, and they show a slow movement of granules. Special cords of protoplasm pass to the tentacle, tooth, lip, and staff-organ. The food is always inclosed in a food vacuole. The nucleus is oval or round, and possesses a distinct nuclear membrane. The tentacle is slightly concave on one side, convex on the other. Its movements are slow; it can be extended, contracted, or rolled up towards its concave aspect, but does not seem to subserve locomotion. It usually attains a length of about half the diameter of the body. Structurally, it consists of a delicate membrane-like investment filled by an extension of the protoplasm.

The concave side is transversely striate owing to a parallel arrangement of the bands of protoplasm1. The tooth, which is yellowish in colour, the lip, and flagellum are also extensions of the protoplasm. The movements of the flagellum are various; it may be retracted and shot out, lashed about, traversed by short rapid, or long and slow undulations. It appears to have periods of quiescence.

1 The tentacle is said by Vignal to resemble physiologically a muscle. It contracts at the opening and closure of a constant current, and passes into tetanus lasting 3-4 minutes under an interrupted current. It ceases to contract spontaneously when poisoned by curari, but remains electrically excitable. See Vignal, Arch, de Physiol, norm, et patholog. (2), v. 1878.

Noctiluca passes into a resting condition but forms no cyst. The tentacle is absorbed, the mouth closed, the peristome, etc. obliterated, but the central mass of protoplasm persists. How the organs in question are regenerated is not known to a certainty 1. Reproduction is by fission and gemmation. It is possible that the former may occur in an ordinary individual, but as a rule the tentacle is thrown off, and all the organs with the exception of the mouth obliterated. The animal becomes elongated transversely, the nucleus first divides with mitosis, then the central mass of protoplasm, and finally a median furrow cuts the two halves asunder. A new tentacle begins to bud out before separation is complete; the peristome and other organs are also reconstituted. Gemmation is often, perhaps always, preceded by conjugation, which takes place by the fusion of two ordinary or resting individuals. The two always come into contact by the peristome or the spot where the peristome lay. The tentacles of the ordinary individuals are said to be thrown off.

In gemmation" the central mass of protoplasm forms a superficial projection, the nucleus divides, and then the projection; and by continuous binary fission, carried out in the same way, a number of spores, amounting to 256 or more generally 512 (circa), take origin. The branching cords of protoplasm are retracted during the process, and the nuclei enter the dividing projections as soon as they number 64. The ripe spore is attached to the parent by a slender pedicle and it developes a flagellum before detachment. It is .076 - .020mm. in length, and viewed laterally has the shape of a coffee bean with one end, the anterior, broad, and the other, or posterior, pointed, with one aspect, the dorsal, convex, and the other, or ventral, slightly concave. The concavity is bordered on each side by a ridge, the two ridges meeting posteriorly (=staff-organ?). Its anterior end is crossed by a furrow from which springs the flagellum, and, according to Cienkowski, a thick rod-like process ( = tentacle?). The flagellum is 6-7 times the length of the body, and is an organ of locomotion. There is a bean-shaped nucleus, and a slowly contractile vacuole (Robin); the protoplasm is homogeneous. The further development of the free spore is not known.

It probably passes into the adult by direct growth 2.

Noctiluca is a voracious animal and floats on the surface of the sea. It is phosphorescent when the water is disturbed, and it appears to emit sometimes a constant feeble light. The seat of the phosphorescence is said to be the superficial layer of protoplasm.

1The Pyrocystis pseudonoctihua of Wyville Thomson and Murray, is probably a resting Noctiluca; and P. fusiformis a similar condition of an unknown Cystoflagellate. See Butschli, Protozoa, pp. 1061-2, and Wyville Thomson and Murray, P. R. S. xxiv. 1876, p. 533, PL 21.

3Cienkowski describes some abnormal forms of attached spores which favour this supposition; see A. M. A. ix. p. 56.