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SEJANUS—SELACHIANS

SEJANUS, LUCIUS AELIUS, favourite and minister of the Emperor Tiberius. He was the son of Seïus Strabo, prefect of the praetorians, and was adopted into the Aeolian gens. After his father's departure from Rome to take up the governorship of Egypt, Sejanus was made into prefect in his stead. He gained the confidence of Tiberius, and, supported by the praetorians, whom he concentrated in a camp on the Viminal Hill, became virtually ruler of Rome. But he aimed still higher, and determined to put all the members of the royal house out of his way. Having removed Drusus (the son of Tiberius) by poison, he persuaded the emperor to retire to the island of Capreae. The death of Drusus was followed some years later by those of Agrippina (the wife of Germanicus) and her sons Drusus and Nero. Tiberius at last saw through his designs, and caused Sejanus to be put to death (A.D. 31).

Tacitus, Annals, iv. 1, 2, 3, 8, 39-59, 74, v. 6-9; Suetonius, Tiberius, 62; Dio Cassius lvii. lviii.; Juvenal x. 65-86; J. Jügl. Vita Aelii Sejani (1882), with notes giving full references to authorities; J. C. Tarver, Tiberius the Tyrant (London, 1902), chap. xvii.

SEKONDI, a port on the Gold Coast in 4° 57′ N., 1° 42′ W., and 167 m. by rail S. by W. of Kumasi. Pop. (1908) about 5000, of whom some 200 were whites. Sekondi is one of the old trading stations on the Guinea coast, and Fort Orange was built here by the Dutch about 1640, the English later on building another fort near by. In 1694 the Dutch fort was plundered by the Ahanta, who in 1698 burnt the English fort. It was not rebuilt, and it was not until 1872 that the place became definitely British. The town was of comparatively little importance until it was chosen as the sea terminus of the railway serving the gold-mining districts and Ashanti. The railway reached the Tarkwa, goldfields in 1901 and the Obuassi mines in 1902. From that date Sekondi became the chief port of the Gold Coast colony, gold, rubber and timber being the principal exports. In 1908 the total trade of the port was £2,121,420. There is no sheltered harbour, but at the landing place are piers provided with cranes. Landing is effected in lighters, ships anchoring in the roadstead half a mile from the shore. The public buildings include Fort Orange, a church, court-house, government offices and hospital. The mean temperature is about 79° F.; the rainfall about 40 in. a year. The climate is unhealthy for Europeans, but by the reclamation of the neighbouring lagoons its sanitary condition has been improved. Sekondi is governed by a municipality, created in 1905. It is in telegraphic communication with Europe by submarine cable, and is served by British, German and Belgian lines of steamers.

SELACHIANS, or Elasmobranchii, a subclass of fishes, including the various kinds of Sharks and Rays.

Structural Features.—The general shape is somewhat spindle like in the Sharks, while in the Rays—in correlation with the ground-feeding habits—the body has become greatly depressed. Departures from the normal are seen in the Hammerheads (Sphyrna), where the sides of the head are so produced as to give a hammer shape, and in the Saw-fishes (Pristis), where the head is prolonged forwards as a greatly elongated flattened rostrum. In regard to the fins, the tail is heterocercal in the adults of living forms, except in Chlamydoselachus, where the protocercal condition persists; the pectoral fins are greatly enlarged in the Rays, in which movement is effected mainly by the passage backwards of waves of flexure along the pectoral fins; the pelvic fins in the last-named fishes have their hinder portions modihed in the male to form special copulatory organs, the myxipterygia or “claspers.”

The mouth opening is a ventrally placed crescentic slit except in Chlamydoselachus, where it is nearly terminal. The olfactory organs, lying in front of the mouth, are widely open to the exterior, and in some cases are connected with the mouth by oronasal grooves. The spiracular opening frequently retains in the adult an opening to the exterior behind or below the eye. In the Rays it is used mainly for inspiration. The post-spiracular clefts open freely to the exterior, each guarded by a flap-like extension of its anterior margin which serves as a valve to allow water to pass only in one direction, viz. outwards. In the Holocephali the anterior flap, that arising from the hyoid arch, is greatly enlarged so as to form an operculum covering over all the clefts lying posterior to it.

The postspiracular clefts are usually five in number, but six in Chlamydoselachus and Notidanus griseus, and seven in N. cinereus. The gill lamellae are strap-like and attached by their edges to the gill septa. Fully developed lamellae are present on the anterior wall o the hyobranchial clefts and vestigial lamellae on the anterior wall of the spiracle where they form the “pseudobranch.”

In the Basking Shark Cetorhinus the pharyngeal openings of the gill clefts are guarded by series of long slender rods—the greatly elongated representatives of the small conical “gill rakers” found in this position in other fishes. These structures form a sieve-like arrangement for preventing the minute creatures (plankton) upon which this shark feeds from passing out through the gill clefts.

There appears to be no representative of the lung or swimbladder, and there are no pyloric caeca. The intestine is provided with a spiral valve in its interior which varies in character in different forms (1). A glandular caecum—the rectal caecum—opens into the dorsal side of the rectum. In regard to the coelomic spaces the Selachians exhibit the interesting feature that the pericardiac cavity is in the adult in communication with the general splanchnocoele by an open channel sometimes forked at its posterior end. This communication apparently arises secondarily and is not due to a persistence of the embryonic communication (2). In the case of Torpedo and in the ordinary Rays certain portions of the muscular system are converted into electrical organs. In the Skates and Rays the electrical disturbance is relatively small—imperceptible by human beings—but in Torpedo it is very considerable. No doubt the electric organs subserve a defensive function.

The kidney of the adult is a mesonephros. The pronephros is never functional, though it appears in a vestigial form in the embryo. The mesonephros shows a division into a broader posterior portion which alone is renal in function, and a slender anterior portion which in the male subserves a genital function. The female genital duct is a typical Müllerian duct having at its anterior end a wide coelomic funnel and lined by glandular epithelium whose secretion forms adventitious coats round the egg during its downward passage. The spermatozoa find their way to the cloaca by way of the mesonephric duct, the hinder portion of which is dilated to form a vesicula seminalis. The urino-genital sinus—formed by the fusion of the mesonephric ducts at their hinder ends—projects forward as a pair of pockets (the so-called sperm sacs).

The skeleton of the Selachian shows remarkably archaic features, inasmuch as the internal skeleton is entirely cartilaginous; the bony or placoid skeleton retaining its primitive superficial position and not showing in any part a tendency to sink or spread inwards for the reinforcement of the cartilaginous skeleton. The vertebral column is of the chordacentrous type, although in some of the more archaic of known fossil forms (Pleuropterygii, Ichthyotomi, Acanthodei, Hybodus) the chondrified secondary sheath of the notochord apparently retained in the adult the unsegmented condition. The same holds for the Holocephali and for the hinder part of the vertebral column of the existing Chlamydoselachus. The centra are usually, if not always, strengthened in the adult by the deposition of lime salts in the intercellular matrix: such calcified cartilage must be carefully distinguished from true bone. The arrangement of the calcified tracts shows differences which are of taxonomic importance.

EB1911 Selachians - vertebra diagrams.jpg

From Zittel's Handbuch der Paläontologie, by permission of Herren R. Oldenbourg, Publishers, Munich.

Fig. 1.—Diagrammatic transverse sections to illustrate the Cyclospondylous (A), the Tectospondylous (B) and the Asterospondylous (C) type of vertebra.

d, d′, d″, Calcified tracts. h.a, Haemal arch.
ex.m, Primary sheath. n.a, Neural arch.

In the cyclospondylous type (fig. 1, A) the calcified tract has the form of a double cone—of the wall of a dice-box—and in the transverse section appears as a simple circle (Palaeospinax, Acanthias, Scymnus). In the tectospondylous (fig. 1, B) type, additional calcified tracts are developed outside and concentric with the original double cone (Batoidei), while in the asterospondylous (fig. 1, C) type the additional calcification takes the form of longitudinally arranged plates radiating outwards from the original double cone, so as to produce a star-like appearance in cross section (Scyllium, Lamna). Eventually in the adult the calcification may extend from the special tracts above mentioned throughout the whole centrum. In certain cases (Carchariidae, &c.) the transverse section of the centrum is modified by its surface becoming indented by the ingrowth of cartilage tracts (calcified or not) situated external to the primary sheath, thus producing an appearance something like a Maltese cross.

The arch elements of the vertebral column have lost in variable degrees the numerical correspondence with the centra which they possibly once possessed. The same applies to the relations of the