veloped; and in both alike the compound corallum may be regarded as a variously-formed aggregate of "corallites," similar in their fundamental structure to the simple corallum.

On the other hand, the corallum of the Rugosa exhibits the following more striking points of difference as compared with that of the Zoantharia sclerodermata :-(1) The septa appear to be primitively developed in four systems, instead of six or five. Sometimes the adult corallum (as in Stauria) exhibits the four primitive septa in a pre-eminently developed condition, but this is not commonly the case. (2) The septa are rendered more or less irregular in their arrangement by the presence of a curious vacant space (sometimes three or four), which is known as the "fossula (fig. 76, B, f), and which appears to take the place of one of the primitive four septa. (3) When the septa are well developed, they generally present themselves in the adult as of two sizes only, a larger and a smaller (fig. 65, B). (4) Tabula are usually present, in conjunction with the septa. (5) The compound coralla possess no true cœnenchyma, and one of their commonest modes of increase is by means of "calicular gemmation."

Recently it has been shown that some very abnormal Rugose corals were provided with a lid or operculum, closing the mouth of the calice. In the genus Calceola (fig. 78), formerly referred to the Brachiopoda, and very

abundant in certain parts of the Devonian System, the operculum consisted of a single valve or piece. In Gonophyllum four valves were present, and in Cystiphyllum prismaticum there were four or more valves in the operculum. It is worthy of notice that some recent corals (species of Primnoa, Paramuricea, and others) exhibit also a more or less complete operculum. The calices of Cryptohelia pudica (one of the Hydroid group of the Stylasterida) are also protected by a calcareous lamina in front of each.

According to Professor Agassiz, the Rugosa and the Tabulate division of the Zoantharia ought not to be considered as be

longing to the Actinozoa, but should be placed amongst the Hydrozoa. This radical change cannot, however, be accepted unless upon the production of much more evidence than has as yet been brought forward in its favour. One strong argument against this view is to be found in the fact that the typical Rugose corals possess well-developed septa - structures which, if they do not absolutely imply the existence of mesenteries, are, at any rate, unknown in any living Hydrozoön, unless Millepora be referred to this class (and in this genus only rudimentary septa are present). At present it is not possible to speak definitely as to the systematic position of the Rugosa, but they appear to form a natural and distinct group, intermediate in many respects between the Zoantharia and the Alcyonaria.

The Rugosa are divided into the following families :

1. STAURIDE: Septa well developed, extending from the bottom to the top of the visceral chamber, and showing a conspicuous quaternary arrangement. Dissepiments are present, and there is a central tabulate area. Genera-Stauria, Polycalia, Metriophyllum, Holo

cystis, Conosmilia.

2. CYATHAXONIDÆ : Corallum simple, with a deep calice; septa well developed, the four primary septa not predominantly developed; no dissepiments or tabulæ. Genera-Cyathaxonia, Guynia, Haplophyllia.

3. CYATHOPHYLLIDE: Corallum simple or compound; septa well developed, but not so completely so as in the two preceding groups; the four primitive septa not pre-eminently developed; tabulæ always, and dissepiments generally, present. Genera-Zaphrentis, Amplexus, Cyathophyllum, Heliophyllum, Omphyma, Lithostrotion, Lonsdaleia, Clisiophyllum, &c.

4. CYSTIPHYLLIDE: Corallum simple or rarely compound; wall complete; septa rudimentary; visceral chamber with small convex vesicles formed by a combination of tabulæ and dissepiments; sometimes an operculum. Genera― Cystiphyllum, Goniophyllum, Rhizophyllum, Calceola.

CHAPTER XVI.

CTENOPHORA.

ORDER IV. CTENOPHORA.-The Ctenophora comprise "transparent, oceanic, gelatinous Actinozoa, swimming by means of ctenophores, or parallel rows of cilia disposed in comb-like plates. No corallum" (Greene).

The members of this order are all free-swimming organisms, and they are placed by many amongst the Hydrozoa, from which, however, they appear to be clearly separated by the possession of a differentiated digestive sac, as well as by their analogies with the Actinozoa, and their generally superior degree of organisation.

Pleurobrachia (Cydippe, fig. 79) may be taken as the type of

Fig. 79.-Ctenophora. Pleurobrachia pileus.

the order, the structure of all being similar to this in essential points. Pleurobrachia possesses a transparent, colourless, gelatinous, melon-shaped body, or "actinosoma," in which the two poles of the sphere are termed respectively the "oral" and "apical," and the rest of the body constitutes the interpolar region. At the oral pole is the transverse mouth, bounded by lateral, slightly protuberant margins. "Eight meridional bands, or ctenophores' bearing the comb-like fringes, or characteristic organs of locomotion, traverse at definite intervals the interpolar region, which they divide into an equal number of lune-like lobes, termed the actinomeres ;' but this division of the body does not extend into the immediate vicinity of the poles, before reaching which the ctenophores gradually diminish in diameter, each terminating in a point" (Greene). The normal number of the ctenophores is eight (four or twelve in some other forms), and each consists of a band of surface elevated transversely into a number of ridges, to each of which a fringe of cilia is attached, so as to form a comb-like plate. The cilia in the middle of these paddle-like transverse ridges are the longest, and they gradually

diminish in length towards the sides, so that the form of each comb is somewhat crescentic. Beside the comb-like groups of vibratile cilia, Pleurobrachia is provided with two very long and flexible tentacular processes, which are fringed on one side with smaller cirrhi. These filamentous processes arise each from a sac, situated on one of the lateral actinomeres, within which they can be completely and instantaneously retracted at the will of the animal.

The mouth of Fleurobrachia (fig. So, a) opens into a fusiform, digestive sac, or stomach (6), the lower part of which is provided with brown cells, supposed to discharge the functions of a liver. The stomach opens below into a shorter and wider cavity (c), termed the "funnel," from which two canals diverge in the direction of the vertical axis of the organism, to open at the "apical pole." These canals are known as the "apical canals" (e), and their apertures as the "apical pores."

Fig. 80.-Morphology of Ctenophora. 1. Diagrammatic transverse section of Pleurobrachia. b Digestive cavity; ii Primary radial canals; kk Secondary radial canals; 17 Tertiary radial canals; g Tentacle.

2. Longitudinal section of Pleurobrachia. a Mouth; Digestive cavity; c Funnel; dd Paragastric canals; e e Apical canals; Ctenophoral canal; g Tentacle; Ctenocyst. (After Greene.)

Of

From the funnel two other pairs of canals are given off. these, one pair-known as the "paragastric canals"-turns upwards, one running parallel to the digestive sac on each side (d), and "terminating cæcally before quite reaching the oral extremity." The second pair of canals (2)-the so-called "radial canals"-branch off from the funnel laterally, each dividing into two, and then again into two, as they proceed towards the periphery of the body. Thus the two "primary radial canals produce four "secondary" canals (k), and these, in turn, give rise to eight "tertiary" radial canals (/), which finally terminate by opening "at right angles into an equal number of longitudinal vessels, the 'ctenophoral canals (),

whose course coincides with that of the eight locomotive bands. These canals end cæcally both at their oral and apical extremities" (Greene). The whole of this complex canalsystem is lined by a ciliated endoderm, and a constant circulation of the included nutrient fluid is thus maintained.

Immediately within the apical pole is situated a small cyst or vesicle, supposed to be an organ of sense, and termed the "ctenocyst" (h). In structure the "ctenocyst" consists of a spherical vesicle, lined with a ciliated epithelium, and filled with a clear fluid, which contains mineral particles, probably of carbonate of lime. Just beneath the ctenocyst is a cellular mass, which has been described as giving off eight filaments running along the ctenophores, and is generally believed to be a nervous system. Eimer denies that the central ganglionic mass is nervous, but describes a plexiform nervous system. The reproductive organs of Pleurobrachia are in the form of folds, containing either ova or spermatozoa, and situated beneath the endodermal lining of the ctenophoral canals, one on each side.

The embryo Pleurobrachia is at first rudely cylindrical in form, a belt of cilia passing round the middle of its body. This soon breaks up into two lateral groups, which eventually disappear altogether. The primitive ctenophores are four in number, each ultimately breaking up into two.

As regards the homologies between Actinia and Pleurobrachia, the following may be quoted from Professor Greene :

"If now a comparison be made between this nutrient system" (the canal system of the Ctenophora) "and that of Actinia, the digestive sacs of the two organisms are clearly seen to correspond in form, in relative size, and mode of communication with the somatic cavity. The funnel and apical canals of Pleurobrachia, though more distinctly marked out, are the homologues of those parts of the general cavity, which in Actinia are central in position, and underlie the free end of the digestive sac. So also the paragastric and radial canals may be likened to those lateral portions of the somatic cavity of Adinia which are not included between the mesenteries. Lastly, the ctenophoral canals of Pleurobrachia and the somatic chambers of Actinia appear to be truly homologous, the chief difference beween the two forms being, that while in the latter the body-chambers are wide and separated by very thin partitions, they are in Pleurobrachia reduced to the condition of tubes; the mesenteries which intervene becoming very thick and gelatinous, so as to constitute, indeed, the principal bulk of the body." The "apical" canals, again, by which the di