1911 Encyclopædia Britannica/Caecilia
|←Caducous||1911 Encyclopædia Britannica, Volume 4
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CAECILIA. This name was given by Linnaeus to the blind, or nearly blind, worm-like Batrachians which were formerly associated with the snakes and are now classed as an order under the names of Apoda, Peromela or Gymnophiona. The type of the genus Caecilia is Caecilia tentaculata, a moderately slender species, not unlike a huge earth-worm, growing to 2 ft. in length with a diameter of three-quarters of an inch. It is one of the largest species of the order. Other species of the same genus are very slender in form, as for instance Caecilia gracilis, which with a length of 2¼ ft. has a diameter of only a quarter of an inch. One of the most remarkable characters of the genus Caecilia, which it shares with about two-thirds of the known genera of the order, is the presence of thin, cycloid, imbricate scales imbedded in the skin, a character only to be detected by raising the epidermis near the dermal folds, which more or less completely encircle the body. This feature, unique among living Batrachians, is probably directly inherited from the scaly Stegocephalia, a view which is further strengthened by the similarity of structure of these scales in both groups, which the histological investigations of H. Credner have revealed. The skull is well ossified and contains a greater number of bones than occur in any other living Batrachian. There is therefore strong reason for tracing the Caecilians directly from the Stegocephalia, as was the view of T.H. Huxley and of R. Wiedersheim, since supported by H. Gadow and by J.S. Kingsley. E.D. Cope had advocated the abolition of the order Apoda and the incorporation of the Caecilians among the Urodela or Caudata in the vicinity of the Amphiumidae, of which he regarded them as further degraded descendants; and this opinion, which was supported by very feeble and partly erroneous arguments, has unfortunately received the support of the two great authorities, P. and F. Sarasin, to whom we are indebted for our first information on the breeding habits and development of these Batrachians.
The knowledge of species of Caecilians has made rapid progress, and we are now acquainted with about fifty, which are referred to twenty-one genera. The principal characters on which these genera are founded reside in the presence or absence of scales, the presence or absence of eyes, the presence of one or of two series of teeth in the lower jaw, the structure of the tentacle (representing the so-called "balancers" of Urodele larvae) on the side of the snout, and the presence or absence of a vacuity between the parietal and squamosal bones of the skull. Of these twenty-one genera six are peculiar to tropical Africa, one to the Seychelles, four to south-eastern Asia, eight to Central and South America, one occurs in both continental Africa and the Seychelles, and one is common to Africa and South America.
These Batrachians are found in damp situations, usually in soft mud. The complete development of Ichthyophis glutinosus has been observed in Ceylon by P. and F. Sarasin. The eggs, forming a rosary-like string, are very large, and deposited in a burrow near the water. The female protects them by coiling herself round the egg-mass, which the young do not leave till after the loss of the very large external gills (one on each side); they then lead an aquatic life, and are provided with an opening, or spiraculum, on each side of the neck. In these larvae the head is fish-like, provided with much-developed labial lobes, with the eyes much more distinct than in the perfect animal; the tail, which is quite rudimentary in all Caecilians, is very distinct, strongly compressed, and bordered above and beneath by a dermal fold.
In Hypogeophis, a Caecilian from the Seychelles studied by A. Brauer, the development resembles that of Ichthyophis, but there is no aquatic larval stage. The young leaves the egg in the perfect condition, and at once leads a terrestrial life like its parents. In accordance with this abbreviated development, the caudal membranous crest does not exist, and the branchial aperture closes as soon as the external gills disappear.
In the South American Typhlonectes, and in the Dermophis from the Island of St Thomé, West Africa, the young are brought forth alive, in the former as larvae with external gills, and in the latter in the perfect air-breathing condition.
REFERENCES—R. Wiedersheim, Anatomie der Gymnophionen (Jena, 1879), 4to; G.A. Boulenger, "Synopsis of the Genera and Species," P.Z.S., 1895, p. 401; R. Greeff, "Über Siphonops thomensis," Sizb. Ges. Naturw. (Marburg, 1884), p. 15; P. and F. Sarasin, Naturwissenschaftliche Forschungen auf Ceylon, ii. (Wiesbaden, 1887-1890), 4to; A. Brauer, "Beiträge zur Kenntnis der Entwicklungsgeschichte und der Anatomie der Gymnophionen," Zool. Jahrb. Ana. x., 1897, p. 389, xii., 1898, p. 477, and xvii., 1904, Suppl. p. 381; E.A. Göldi, "Entwicklung von Siphonops annulatus," Zool. Jahrb. Syst. xii., 1899, p. 170; J.S. Kingsley, "The systematic Position of the Caecilians," Tufts Coll. Stud. vii., 1902, p. 323.
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