1911 Encyclopædia Britannica/Sirenia

by Richard Lydekker

SIRENIA, the name (in reference to the supposed mermaid-like appearance of these animals when suckling their young) of an order of aquatic placental mammals, now represented by the manati (or manatee) and dugong, and till recently also by the rhytina. Although in some degree approximating in external form to the Cetacea, these animals differ widely in structure from the members of that order, and have a totally distinct ancestry.

The existing species present the following leading characteristics. The head is rounded and not disproportionate in size as compared with the trunk, from which it is scarcely separated by any externally visible constriction or neck. Nostrils valvular, separate, and placed above the fore-part of the obtuse, truncated muzzle. Eyes very small, with imperfectly formed eyelids, capable, however, of contraction, and with a well-developed nictitating membrane. Ear without any conch. Mouth of small or moderate size, with tumid lips beset with stiff bristles. General form of the body depressed fusiform. No dorsal fin. Tail flattened and horizontally expanded. Fore-limbs paddle-shaped, the digits being enveloped in a common cutaneous covering, though sometimes rudiments of nails are present. No trace of hind-limbs. External surface covered with a tough, finely wrinkled or rugous skin, naked, or with sparsely scattered fine hairs.

The skeleton is remarkable for the massiveness and density of most of the bones, especially the skull and ribs, which add to the specific gravity of these slow-moving animals, and aid in keeping them to the bottom of the shallow waters in which they dwell, while feeding on aquatic vegetables. The skull presents many peculiarities, among which may be indicated the large size and backward position of the nasal aperture, and the downward flexure of the front of both jaws. The nasal bones are absent, or rudimentary and attached to the edge of the frontals, far away from the middle line; but in some extinct species these bones, though small, are normal in situation and relations. In the spinal column none of the vertebrae are united together to form a sacrum, and the flat ends of the bodies do not ossify separately, so as to form disk-like epiphyses in the young state, as in nearly all other mammals. The anterior caudal vertebrae have well-developed chevron-bones. In one genus (Manatus) there are only six cervical vertebrae. There are no clavicles. The humerus has a small but distinct trochlear articulation at the elbow-joint; and the bones of the fore-arm are about equally developed, and generally welded together at both extremities. The carpus is short and broad, and the digits five in number, with moderately elongated and flattened phalanges, which are never increased beyond the number usual in Mammalia. The pelvis is rudimentary, consisting of a pair of bones suspended at some distance from the vertebral column.

Two kinds of teeth, incisors and molars, separated by a wide interval, are generally present. The former may be developed into tusks in the upper jaw, or may be quite rudimentary. The molars vary much in character. In one genus (Rhytina) no teeth of any kind are present, at least in the adult. In all, the anterior part of the palate, and a corresponding surface on the prolonged symphysis of the lower jaw, are covered with rough horny plates of peculiar structure, which doubtless assist in mastication. The tongue is small and fixed in position, with a surface resembling that of the aforesaid plates. The salivary glands are largely developed. The stomach is compound, being divided by a valvular constriction into two principal cavities, the first of which is provided with a glandular pouch near the cardiac end, and the second usually with a pair of elongated, conical, caecal sacs or diverticula. The intestinal canal is long, and with very muscular walls. There is a caecum, either simple, conical, and with extremely thick walls, as in Halicore, or cleft, as in Manatus. The apex of the heart is deeply cleft between the ventricles. The principal arteries form extensive and complex network-like structures, retia mirabilia. The lungs are long and narrow, as, owing to the oblique position of the diaphragm, the thoracic cavity extends far back over the abdomen. The epiglottis and arytenoid cartilages of the larynx do not form a tubular prolongation. The brain is comparatively small, with the convolutions on the surface of the cerebrum few and shallow. The kidneys are simple, and the testes abdominal. The uterus is bicornuate. The placenta is non-deciduate and diffuse, the villi being scattered generally over the surface of the chorion except at the poles. The umbilical vesicle disappears early. The teats are two, and pectoral or rather post-axillary in position.

In vol. lxxvii. of the Zeitschrift für wissenschaftliche Zoologie Mr L. Freund describes in detail the osteology of the flippers of the dugong as displayed in “sciograph” pictures. These show that the carpus of the adult consists of three large bones. Of the two in the first row, one consists of the fused radiale and intermedium, and the other of the ulnare plus the pisiform and the fifth carpale, the lower bone being composed of the four inner carpalia. In the manati the reduction of the carpus has been carried to a less extent, the radiale being in some instances distinct from the intermedium, while in other cases in which these two bones are fused the four inner carpalia remain separate.

Sirenians pass their whole life in water, being denizens of shallow bays, estuaries, lagoons and large rivers, and not met with in the high seas far away from shore. Their food consists entirely of aquatic plants, either marine algae or freshwater grasses, upon which they browse beneath the surface, as the terrestrial herbivorous mammals do upon the green pastures on shore. To visit these pastures, they come in with the flood-tide and return with the ebb. They are generally gregarious, slow and inactive in their movements, mild, inoffensive, and apparently unintelligent in disposition. Though occasionally found stranded by the tide or waves, there is no evidence that they voluntarily leave the water to bask or feed on the shore. The habit of the dugong of raising its round head out of water, and carrying its young under the fore fin, seems to have given rise, among the early voyagers in the Indian Ocean, to the legendary beings, half human and half fish, in allusion to which the name Sirenia was bestowed by Illiger. The species now existing are few. One species, Rhytina gigas of the North Pacific, was exterminated through the agency of man during the 18th century; and the others, being valuable for their flesh as food, for their hides, and especially for the oil obtained from the thick layer of fat which lies immediately beneath their skin, diminish in numbers as civilized populations occupy the regions forming their natural habitat. The species are confined to the tropical regions of the shores of both sides of the Atlantic and the great rivers which empty themselves into that ocean, and to the coasts of the Indian Ocean from the Red Sea to North Australia.

As regards dentition (or the want thereof) the three modern genera are remarkably different; and while on this and other grounds some writers refer them to as many separate families, by others they are all included in the Manatidae.

Fig. 1.—Skull of African Manati (Manatus senegalensis).
From Mus. Roy. Coll. Surgeons.

In the manatis (Manatus) the incisors, 2/2 in number, are rudimentary, and concealed beneath the horny mouth-plates, and disappearing before maturity. Molars about 20/20, but rarely more than 6/6 present at one time; the anterior teeth falling before the posterior come into use; similar in characters from beginning to end of the series; with square, enamelled crowns, the grinding surface raised into tuberculated transverse ridges. The upper teeth with two ridges and three roots, the lower with an additional (posterior) ridge or heel and two roots. The cervical vertebrae present the anomaly of being reduced to six in number, the usual vertebral formula being C 6, D 15-18, L and C, a 25-29. Rostrum of the skull, formed by the union of the premaxillae in front of the nasal aperture, shorter than the length of the aperture and scarcely deflected from the basi-cranial axis. Tail entire, rounded or shovel-shaped. Rudimentary nails on the fore-limbs. Caecum cleft.

Manatis inhabit the shores of, and the great rivers which empty themselves into, the Atlantic within the tropics. The American (M. australis) and African (M. senegalensis) forms are generally considered distinct species, though they differ but little from each other in anatomical characters and in habits. There is also the small M. inunguis of the Amazon, which has no nails. They are rather fluviatile than marine, ascending large rivers almost to their sources (see Manati).

Fig. 2.—American Manati (Manatus australis).

In the dugong (Halicore) the upper jaw is furnished with a pair of large, nearly straight, tusk-like incisors, directed downwards and forwards, partially coated with enamel. In the male they have persistent pulps, and bevelled cutting edges, which project a short distance from the mouth, but in the female, though they remain through life in the alveolar cavity, they are not exserted, and, the pulp cavity being filled with osteodentine, they soon cease to grow. In the young there is also a second small deciduous incisor on each side above. At this age there are also beneath the horny plate which covers the anterior portion of the mandible four pairs of slender conical teeth lodged in wide socket-like depressions which become absorbed before the animal reaches maturity. The molars are usually 5/5, sometimes 6/6, altogether, but not all in place at once, as the first falls before the last rises above the gum; they are more or less cylindrical in section, except the last, which is compressed and grooved laterally, without distinction into crown and root, increasing in size from before backwards, with persistent pulps and no enamel. The summits of the crowns are tuberculated before wearing, afterwards flattened or slightly concave. Skull with rostrum formed by the union of the premaxillae in front of the nasal aperture, longer than the aperture itself, bending downwards at a right angle with the basi-cranial axis, and enclosing the sockets of the large tusks. Anterior part of the lower jaw bent down in a corresponding manner. Vertebrae: C 7, D 18-19, L and C 30. Tail broadly notched in the middle line, with two pointed lateral lobes. No nails on the fore-limbs. Caecum single. The genus is represented by H. tabernaculi from the Red Sea, H. dugong from the Indian seas and H. australis from Australia. (See Dugong.)

The last genus is represented only by the extinct Rhytina gigas, of Bering Sea, in which there were no teeth, their place being supplied functionally by the dense, strongly-ridged, horny mouth-plates. Premaxillary rostrum about as long as the anterior narial aperture, and moderately deflected. Vertebrae: C 7, D 19, L and C 34-37. Head very small in proportion to the body. Tail with two lateral pointed lobes. Front limbs small and truncated. Skin naked and covered with a thick, hard, rugged, bark-like epidermis. Stomach without caecal appendages to the pyloric cavity. Caecum simple. See Rhytina.

Extinct Sirenia.—In past times the Sirenia were represented by a number of extinct generic types ranging over all the temperate and probably tropical regions, and extending from the Pliocene to the Eocene epoch. In the Pliocene of Europe the group is represented by Felsinotherium, in the Miocene by Metaxytherium, and in the Oligocene by Halitherium; the latter having an acetabular cavity to the pelvis and a rudimentary femur. From Halitherium, which has a somewhat maniti-like dentition, although there are few cheek-teeth, there is a transition through the other two genera to Halicore; Felsinotherium having a large pair of tusk-like upper teeth. In Halitherium milk-molars were developed. In Miosiren, of the Belgian Miocene, the teeth were differentiated into i. 1/1, p. 3/3, m. 4/4. Remains of several early types of sirenians have been obtained from the Eocene deposits of Egypt. The least generalized of these is Eosiren, an animal differing from the modern forms chiefly by the retention of traces of the second and third pairs of incisors and of the canines, and the somewhat less degree of reduction in the pelvis, which has a complete acetabulum for the head of the femur. The front teeth (incisors and canine) have, however, been thrust to the sides of the jaw, possibly to make room for a horny plate on the palate. In the somewhat earlier Eotherium the incisors and canines are larger and occupy the normal position in the front of the jaws; while the pelvis has a closed obturator foramen and a complete acetabulum, suggestive that a functional thigh-bone or femur was still retained. The most primitive member of the group with which we are yet acquainted is the very imperfectly known Prorastomus, from the Eocene of the West Indies, in which a complete and fully differentiated dentition is accompanied by the absence of that deflection of the front part of the jaws which constitutes one of the most striking features of all the foregoing representatives of the order;—a feature which Dr C. W. Andrews has pointed out must be of great value to short-necked, long-bodied creatures feeding on the herbage at the bottom of the water in which they dwell.

The foregoing Egyptian fossil sirenians afford important evidence with regard to the ancestry of the order. Many years ago it was suggested by the French naturalist de Blainville that the Sirenia are related to the Proboscidea. This is supported by the occurrence of the remains of some of the most primitive sirenians with those of the most primitive proboscideans in the Eocene formations of Egypt; confirmatory evidence being yielded by the similarity of the brain and to some extent of the pelvis in the ancestral forms of the two groups. As regards the living members of the two groups, both have pectoral teats, abdominal testes, and a cleft apex to the heart; while the cheek-teeth of the sirenians are essentially of the same type as those of the early proboscideans. There seems also to be a certain similarity in the mode of succession of the teeth in the more specialized members of the two groups, although in the sirenians this specialization has displayed itself in an abnormal augmentation of the number of the teeth, while in the proboscideans, on the other hand, it has taken the form of an increase in the complexity of the individual teeth, especially those at the hinder part of the series. Finally, although the Proboscidea have a deciduate and the Sirenia a zonary nondeciduate placenta, yet there are certain similarities in the structure of this organ in the two groups which may indicate genetic affinity.

Literature.—O. Thomas and R. Lydekker, “On the Number of Grinding-Teeth possessed by the Manatee,” Proc. Zool. Soc. (1897); G. R. Lepsius, “Halitherium schinzi, die fossile Sirene des Mainzer-Beckens,” Abhandl. Mittelrhein. Geol. Vereins (1881 and 1882); O. Abel, “Die Sirenen der mediterranen Tertiärbildungen Österreichs,” Abhandl. k. k. geol. Reichsanstalt, Wien, vol. xix. (1904); and “Über Halitherium bellunense, eine Übergangsform zur Gattung Metaxytherium,” Jahrbuch k. k. geol. Reichsanstalt, Wien, vol. lv. (1905); C. W. Andrews, Descriptive Catalogue of the Vertebrata from the Fayum (British Museum, 1906).  (R. L.*)