Imágenes de páginas
PDF
EPUB

arrangement in the coral polypes, and even in the remarkable Cerianthidæ, the basilar opening into the visceral cavity is part of a water system, and is not intestinal. When, however, the hard parts of some tubular Polyzoa are placed in relation with certain tubular masses said to be skeletons of Actinozoa, the distinction is still very unsatisfactory. Such tubular masses are very common in the paleozoic rocks, in the oolites, and even in the miocene deposits; and the rule has been to consider those as Polyzoa which have no floors or cross-pieces filling up the calibre of the tube here and there. But the late Jules Haime, the most original worker amongst the corals and Bryozoa, described some oolitic tubular masses with cross pieces (tabulæ), as not belonging to the Actinozoa but to the Polyzoa. His early death, so much to be deplored, appears to have prevented his giving a reason for this. There are no recent Polyzoa with tabulate tubules, and it is therefore probable that the decided absence of septa may have been considered of value by Haime in distinguishing the tubules in question from the corallites of Actinozoa. On the other hand, the non-septate tubules of Tubipora musica, a well-known Actinozoan, offer a protest against such a decision. This difficulty has been greatly increased by the dictum of Agassiz, that tabulate Actinozoa are really Hydrozoa.

The Hydrozoa are distinguished from the Polyzoa by not possessing the separate opening for the anus; and from the Actinozoa, by their ovarian structures not opening into the visceral or perivisceral cavities, but externally. The separation of the gastric and generative systems is the essential peculiarity of the Hydrozoa. They are closely allied to the Actinozoa, and form with them the Coelenterata of Frey and Leuckart; but there are some forms, such as Lucernaria and Beröe, which have produced a diversity of opinion as to their Hydroidean or Actinozoan characters. The assertion, by so able an observer as Agassiz, that the Milleporidæ, and therefore the vast assemblage of tabulate corals, are clearly related to the Hydractiniæ, and are true Hydroid Acalephs, strikes at the heart of the received classification, The tubular forms with tabulæ- the Polyzoa of Haime—cannot be classified with the tabulate Madreporaria ; and thus the forms which present the most evident relation with true stony corals, and into whose structures tabulæ enter in a very secondary manner, cannot be placed in any classification with safety.

It is greatly to be regretted that Agassiz has not carried out his researches, and that the drawings which Dana had from him,

and which he declared like Hydrozoa, have not been published.

More or less separable, therefore, from the Polyzoa and Hydrozoa, the vast assemblage of forms called corals-the Actinozoa—is thus distinguished :

1. There is a central mouth encircled by tentacules, and there is not a proper

anal vent. 2. The body is hollowed out to form a single system of communicating cavities, which open finally at the mouth.

3. The generative organs are internal, and are situate within the general cavity.

The varieties of shape, of complexity, and of habits in the animals thus distinguished, are immense, and there is great diversity in the density of the tissues and the hardness of the internal skeleton, as well as in the methods of reproduction. If the Ctenophoræ (Beröe etc.) and the Lucernarias are placed in the position of uncertain beings, a very simple and natural classification of the Actinozoa may be made, and to do this is not very unreasonable, for Green and Huxley reject Lucernaria as one of the Actinozoa, although MM. Milne Edwards and Jules Haime place it in the class in the order Podactinaria, and the great French zoophytologists do not admit the Ctenophoræ amongst the “Coralliaires or Polypes proprement dites."

With these restrictions, the Actinozoa may be divided into those which have the tentacules regularly pinnate, and eight in numberthe Alcyonaria ; and into those whose tentacules are not eight in number, nor pinnate, but simple or irregularly ramose-the Zoantharia.

The order Alcyonaria separates very naturally into three families.

1. Gorgonidæ.-Adherent, and possessing a a calcareous or corneous axis.

2. Alcyonidæ.-Adherent, simple or compound, and with a fleshy corallum.

3. Pennatulidæ.-Free-swimming, or non-adherent, with an axis.

There are also three families of Zoantharia.
1. Sclerobasic forms with a hard axis—Antipatharia.

2. Malacodermic forms without an axis, but more or less fleshy. -Actinidæ.

3. Sclerodermic forms with a calcareous skeleton, and without an axis-Madreporaria.

It will be evident from an examination of this classification that

it is possible to arrange the Actinozoa artificially in the following groups :

Corallum coriaceous, compound or simple, more or less spiculate. -Alcyonidæ. Actinidæ. Corallum, with a hard axis and adherent.—Gorgonidæ.

Antipatharia.

Corallum with a hard axis and free.-Pennatulidæ.

Corallum simple or compound, with a calcareous non-axial skeleton.—Madreporaria.

The number of genera thus associated is very great, and it is increased by those families of the Madreporaria which are only known by their fossil corallites. It will be observed how very correctly the earliest naturalists joined together their Mediterranean corals under one denomination.

A gradual approximation in structure may be traced from the simplest and softest Alcyonida to the densest, most complicated, and faintly membraned Madreporarian. The various families mimic each other, and produce simple, ramose, and aggregated forms, and they all contain species which reproduce by ova and by gemmation. The spiculate tissues of the soft Alcyonidæ are an approach to the hard stems of the Gorgonidæ, and are represented in the delicate reticulations of some of the Porites; and the Telestinæ assume the shape of the branching Actinozoa with a calcareous axis. The simple arrangement of the spicules of Nephthya Chabrolië (Audouin) of the Red Sea, within a rather dense tissue, is noticed in a greater degree in Spoggodes Celosia (Lesson), of the Fijees; and the structure is more fully elaborated in the cylindrical fleshy tube of Paralcyonium, whose spiculiferous walls are evidently for the protection of the soft polypes by which they are produced. Now the step from this tube to the tube of Tubipora musica is not very great in the gradation of structure. To complete the structural chain, some Actinidæ are required well furnished with sclerites, foreshadowing Perforate Madreporaria, but there have been no satisfactory proofs of the existence of such species. The resemblance between the soft tissues of the Madreporaria and the Actinidæ is very great, and the immature stony corals, on their first adhesion to solid substances, shortly after the termination of their embryonic life, are like Actinidæ of corresponding age.

The tube of the Cerianthidæ is not an approximation to the hard part (corallum) of stony corals, for it is produced in a different manner; it consists of the filaments of the peculiar cells which abound in the tissues of the Actinozoa—the nematocysts, aggregated and

joined to masses of sand and stone with a mucous secretion. It is, therefore, a special peculiarity of these sand-dwelling Actinidæ.

Even the free Sea Pen has its manner of life mimicked amongst the Zoantharia, for the Minyadinæ are free-swimming, or rather floating Actinidæ. Formed on the plan of the sea anemones, their bases, instead of resembling the discs of the adherent genera, are dome-shaped, and encroach upon the visceral cavity. A sphincter produces a constriction inferiorly, and a purse-shaped chamber above it. The air contained in this enables the Minyad to float. If the sides of the chamber secreted a dense tissue which could project, the mimetic resemblance between the Alcyonarian and the Zoantharian would be great.

The true stony corals, the Madreporaria, are readily distinguished from all the other families of the Actinozoa, and have their skeletons so distinctly in excess over the soft tissues, that the structures of these important parts are too often neglected by classificatory zoophytologists. Some simple or solitary forms are to be found on our south-western coasts, and the deep sea representative of one of them may be dredged up by hundreds in the north-eastern sea, off Shetland. The Caryophyllia Smithiï (Stokes), of Cornwall, is the littoral form of Caryophyllia borealis (Fleming), of the Shetland seas, and is known as a deep sea coral in the Mediterranean, being called Caryophyllia clavus (Scacchi). The species is found fossil in Sicily, in the pliocene deposits, and its present bathymetrical range is from low spring tides to more than 250 fathoms. It is a complicated piece of structure, even as regards the hard skeleton, and an examination of all the tissues is very instructive. Specimens are to be dredged up, from the size of a pea to that of a cone an inch in height and breadth at the free end.

The species reproduces by ova, which, escaping from the ovaries into the visceral cavity, gain the sea through the mouth. Now, it is most remarkable what a choice, passive in its nature, the ova have for particular bodies upon which to settle and to live. In some spots, every Caryophyllia dredged up is found on the shell of the annelid Ditrupa, and in the Mediterranean a fusiform gasteropod shell is the favourite support. The ciliated ovum touches, adheres, and surrounds the supporting texture more or less, and speedily develops its rudimentary sclerenchyma within the basal membrane.

Sometimes the ova adhere to the outside of the parent; and examples are not very rare of the adhesion of an ovum to the tissues near the parent's mouth. In the first instance, a portion of

the soft structures covering the external surface is removed, and the base of the young coral becomes permanently attached to the costal structures; and in the last, the growing coral destroys the parent. The conico-cylindrical and curved shape of the full-grown Caryophyllia borealis prevents its remaining upright even when attached to the Ditrupa ; doubtless, in the youngest stage of its growth it could exist on the surface of the deep sea ooze without falling over ; but in all probability the upward growth of the coral is attended by an increase in the depth of the fine deposit around it, and the greater part of the form is always embedded and supported. If any coral be removed suddenly and violently from the water, or even if it be subjected to unusual movement in the sea, the soft tissues are so completely retracted, and get rid of so much of their fluid, that only a faint glaze is noticed to cover the dense white skeleton or sclerenchyma. When unusually stimulated, the upper part of the coral, which presents the central mouth and its surrounding disc, covered more or less with expanded tentacules, soon begins to alter in shape; the tentacules retract with greater or less rapidity, and the lips of the mouth are projected on the top of a short neck-like protuberance; and if the stimulation persists, this structure is retracted, and the tentacules appear to be lost in the spaces between the radiating lamellæ (septa), which, like the spokes of a wheel, pass from the centre of the upper star-shaped opening (the calice) to its circumference.

When one of the Caryophylliæ is examined-some care having been taken not to treat it too rudely—the outside, from the base to the margin of the calice, is seen to be covered with a fine membrane, through which the parallel granular projections, the costæ, and the faint intercostal spaces are to be distinguished. This transparent tissue reaches from the adherent base to the free upper ends of the costa, where they join the external ends of the septa : at this spot it is continuous with the structures of the disc. The tentaculiferous disc covers and reaches higher than the calice; it is perforated by the mouth, and supports the concentric rows of tentacules. It is, when the polype is fully expanded, rather higher than the top of the curved septa at the circumference of the calice; so that it bounds superiorly a cavity whose base is the deep hollow of the calice, its central axis the columella, pali and the radiating septal plates. A soft tissue, covering the septa and the wall between them, joins that just mentioned as covering the outside close to the margin of the calice, and becomes the short, external, and superior boundary of the sub-discal cavity. When the mouth is open, a white mass

« AnteriorContinuar »