Encyclopædia Britannica, Ninth Edition/Cœlentera
According to Van Beneden, R. Leuckart, and some others, the Sponges also have their place among Cœlentera,—a view which has of late years received much support in consequence of the profounder study of the calcarious[1] sponges begun by Miklucho-Maclay and diligently followed up by Haeckel. There is much to be said in favour of regarding the sponges as an aberrant (and, at the same time, degraded) coelenterate class, but, for the present, it will be well to treat them as a group apart.
It is usual to consider the Cœlentera (with or without the sponges) as a primary group, or sub-kingdom, of animals; and a high authority has stated that the institution of this group has been the greatest improvement in the arrangement of the animal kingdom effected since the time of Cuvier. But, should we so interpret the results of certain recent embryological inquiries as to throw the Cœlentera into one great division along with all the higher invertebrates, such a mode of treatment would reduce Cœlentera to the rank of a province.
Name.—The word Cœlentera (or rather its German equivalent) first occurs on page 38 of Beiträge zur Kenntniss wirbelloser Thiere, von Frey und Leuckart, Braunschweig (Vieweg), 1847.[2]
Here it should be mentioned that Burmeister (Zoonomische Briefe, Zweiter Thail, p. 279) has given the same name to a very different group of animals. He denotes by it the majority of the nematoid worms, placing in a separate section (Amorphocœla) Gordius and its allies, whose alimentary canal is more or less atrophied. In this sense Cœlentera is nearly equivalent to Cœlelminthes of Cuvier.
Cœlentera is derived from κοίλος (hollow) and ἔντερον (intestine or viscus).
Definition.—Allowing for the difficulty of expressing modern scientific concepts by compounds formed from words in common use, the meaning of which needs to be somewhat stretched, this etymology guides us to the definition of the Cœlentera as animals having a conspicuous alimentary canal, which, with its prolongations, occupies the whole interior of the body,[3] and does the work of a vascular as well as of a digestive system. It is not true to add, however, that the Cœlentera are invariably destitute of cavities comparable (morphologically) to the blood vessels, perivisceral spaces, and other serous passages of the higher animals. Such cavities, hitherto usually overlooked, undoubtedly exist in some cases, as appears from the investigations of Metschnikoff,[4] Eilhard Schulze,[5] and others.
The wall of the body in the Cœlentera has the same fundamental composition as among the higher animals, and exhibits various degrees of differentiation.[6] Inner and outer layers of epithelial tissue, splinted by connective tissue (in close relation with which we usually find muscular fibres), are always developed.
Neither the absence of nervous tissues nor the presence of those curious microscopic organs known as thread-cells can henceforth be enumerated among the characters common to and distinctive of Cœlentera. Though a nervous system remains to be discovered in many, it certainly exists in some; and in yet other cases, where anatomical evidence is wanting, its presence may reasonably be conjectured from purely physiological data.
Most, if not all, Cœlentera have thread-cells; but these exist likewise in other organisms, notably in certain mollusks which were formerly supposed to derive them from the cœlenterate animals on which they preyed.
The plant-like aspect of many Cœlentera arises in two ways. In the simple (not compound) cœlenterates, such as most sea-anemones, the tentacles or prehensile appendages are so arranged as to simulate, when not too closely inspected, the petals of ordinary flowers (particularly flowers with numerous narrow petals, e.g., Mesembryanthemum) or the strap-shaped corollas of composite plants, like dahlias. In the compound species buds and branches are formed, marking changes in direction of growth; and hence those wonderful phytoid aggregates which for so many centuries puzzled naturalists.
Affinities.—The nearest relations of Cœlentera are undoubtedly the Echinoderms, whose remarkable vascular system is developed from one or more rudiments primarily formed as diverticula of the alimentary canal. The Cœlentera exhibit, even more perfectly than the echinoderms, a radiated arrangement of their parts, and, to a lesser degree, have this primitive disposition controlled by a superinduced bilateral symmetry. On the other hand the affinities of cœlenterates to worms, save through the echinoderms, are very obscure.[7]
Of animals inferior to the Cœlentera in complexity of structure their nearest reputed allies are the Infusoria. We are not yet able, however, to demonstrate the existence of any relationship of this kind, in spite of all that has been urged in its favour by Claparede, Greef, and other eminent anatomists.
- ↑ See Die Kalkschwämme, von Ernst Haeckel, Berlin (Reimer) 1872.
- ↑ See further another work by Leuckart, Ueber die Morphologie und die Verwandtschaftsverhältnisse der wirbellosen Thiere, ibid., 1848; and the valuable " Bericht " contributed by the same writer to the Archiv fur Naturgeschichte from that date to the present; also his university programme, entitled—De Zoophytorum et historia, el dignitate systematica, Lipsiae, 1873.
- ↑ The doubts suggested on this point by R. Leuckart (Bericht f. 1868–9, p. 188), in opposition to the views of Noschin, Semper, and Kowalewsky, may now at length be regarded as set at rest by the appearance of the last-named writer's recent Memoir on the Development of the Cœlentera. This indispensable work has unfortunately been printed in the Russian language, but the reader may consult its figures, in conjunction with the excellent German abstract, by Hoyer, in the second vol. of the Jahresberichte of Hofmann and Schwalbe.
- ↑ " Studien über die Entwickelung der Medusen und Siphonophoren," in Zeitschr. f. wiss. Zool., xxiv. Band, p. 73.
- ↑ Uber den Bau von Syncoryne Sarsii, Leipzig (Engelmann), 1873.
- ↑ Almost the only comprehensive details on this subject which we possess are contained in the Russian memoir by Kowalewsky, already referred to.
- ↑ On the mutual relations of these groups, consult the concluding part of an essay by A. Goette—“Vergleichende Entwickelungsgeschichte der Comatula mediterranea,” in Archiv für Mikroskopische Anatomie, xii. Band. 1876.