As has been already said, nothing is
known of the placentation of this small but important group
, all the existing forms of which are aquatic in
their habits, frequenting great rivers and their estuaries; and
are devoid of hind-limbs, while the integument of the caudal
end of the body is produced into a flattened horizontal fin.
No dorsal fin is ever present. The demarcation between the
head and neck is but obscurely marked, and the fore-limbs are
converted into paddles, upon which only rudimentary nails are
developed. Scanty bristles cover the surface of the body.
The snout is fleshy and tumid, and the valvular nostrils, which
are perfectly distinct from one another, are situated considerably
above its termination. There is a well-developed third
eyelid, the pinna of the ear is absent, and the mammae are
thoracic; a circumstance which has probably not a little contributed
to the origination of the myths respecting the existence
were formerly united with the Whales and
Porpoises as Cetacea herbivora
. But their organization differs
from that of the true Cetaceans in almost every particular,
while they are closely allied with the Ungulata
The cervical vertebrae are reduced to six in one genus- Manatus
. The bodies of these vertebrae are always compressed
from before backward, but they are never all anchylosed
together (it is rare for any of them to be thus
united), and the second has a distinct odontoid process. The
dorsal vertebrae have broad and depressed spines, and may be
as many as seventeen or eighteen in number, while there are
not more than three lumbar vertebrae; and the hindermost of
these even is probably to be regarded as sacral. There are
twenty or more caudal vertebrae, the terminal ones being not
polygonal, but depressed, with well-developed processes.
The zygapophyses of successive vertebrae articulate together
in the dorsal region; but, in the lumbar and caudal regions,
the postzygapophyses disappear and the prezygapopbyses
are small, and neither overlap, nor embrace, the spine
of the antecedent vertebra. The posterior moiety of the spine
thus acquires considerable flexibility. There is no true sacrum,
the vertebra called "sacral" being only determined as such
by its connection with the rudimentary pelvis. Strong subvertebral
chevron-bones are placed beneath the interarticular
cartilages of the caudal vertebra. The heads of the ribs
articulate with the centra of all the vertebrae. The bodies of
the ribs are very thick, rounded, and have a remarkably dense
and laminated structure. The narrow and elongated sternum
is an undivided mass of bone, and is connected by ossified
sternal ribs with the anterior three pairs of vertebral ribs.
In the skull the elongated and subcylindrical form of the
cranial cavity is worthy of notice, as it strongly contrasts with
the form of the brain-case in the Cetacea
. The supraoccipital
is very large and slopes upward and forward a long way on to
the upper surface of the skull; but it does not separate the
parietal bones; which, as usual, unite in the sagittal suture.
The frontals nre prolonged into broad supraorbital processes.
The nasal bones are abortive, and, in the dry skull, the external
nares are very wide, and look upward. The tympanic bone is
a thick hoop, anchylosed with the periotic bones, and readily
comes away from the skull with them. The zygoma is
enormously stout. The premaxillae constitute a large portion
of the boundary of the gape; and the lower jaw has a high
ascending portion, with a large coronoid process.
The scapula has a distinct spine occupying the ordinary
position. There are no clavicles. The humerus has its distal
end fashioned into articular surfaces, upon which the radius
and ulna are freely movable. The pollex is rudimentary, and
the other digits have no more than three phalanges each.
The pelvis is rudimentary, the bones which represent the ossa innominata
being connected by their proximal ends with
the transverse processes of the last of the precaudal vertebrae.
They are disposed vertically to the axis of the body. No
trace of the hind-limbs has been observed in any of the
The premaxillary region of the palate, and the corresponding
surface of the mandible, are coated with mammillated and
rugose horny plates formed of hardened epithelium; and, in
the extinct genus Rhytina
, these plates were
the only masticating organs,
as there were no teeth. In Halicore
Dugong), there are teeth which have no vertical successors,
form no roots, and are devoid of enamel; while, in Manatus
there are milk-molars, and the grinding teeth are enamelled,
and present crowns with double transverse ridges.
|Fig. 103. - Dorsal view of the heart of a Dugong (Halicore), its cavities being laid open. - R
v., right ventricle; L.v., left ventricle. V. c. s. s., left superior vena cava. V. c. s. d.. right superior vena cava. V. c. i., vena cava inferior. F. o. v.. the inner end of a caecal
diverticulum of the right auricle, into which a style is introduced, and which represents
the foramen ovale. O, the auricular septum.
The adult Manatee has no incisors. In the Dugong there
are no incisors in the mandible of the adult. The male has
two tusk-like incisors which project from their sockets in the
premaxillae; while, in the female, the tusks remain concealed
in their alveoli.
In the foetal state, both Halicore
in the mandible as well as in the premaxillae.
The stomach is divided into two portions by a median constriction,
and its cardiac end is provided with a peculiar gland.
Its plyoric end, in some species, gives off two caeca.
There is a caecum at the junction of the large and small intestine.
Salivary glands are well developed. The apical portion of
the septum ventriculorum
is deeply cleft, so that the
ventricles are separated from one another through about half
There are two superior cavae and a Eustachian valve. Extensive arterial
and venous retia mirabilia
are developed in Manatus
. In consequence of the great length of the thoracic
region and the brevity of the sternum, the diaphragm takes a
very unusual course, extending very obliquely from before
backward, and causing the upper part of the thoracic cavity to
extend posteriorly over almost the whole of the abdomen.
The greatly-elongated lungs fill this part of the thoracic chamber,
while the broad heart lies in its anterior and sternal portion.
The arytenoid cartilages are not prolonged as in the Cetacea.
A broad and high epiglottis is capable of covering the
There is no third bronchus.
The cutaneous muscle is largely inserted into the humerus,
and the sub-caudal muscles extend forward as far as the posterior
lumbar vertebrae. The chief muscles of the antibrachium
and manus are present.
The male Sirenia
possess vesiculae seminales. The uterus
There are two living genera of Sirenia
—the Dugong (Halicore)
, which is found upon the shores of the Indian
Ocean and of Australia; and the Manatee (Manatus)
is confined to the South American and African borders of the
A third genus, Rhytina
, which had a coriaceous integument
almost devoid of hair, and possessed no teeth, abounded
in Behring's Straits less than a century ago. It is now altogether
The Miocene genus, Halitherium
, appears to have possessed
distinct, though small, hind-limbs.