We perceive that the internal construc tion of the organs of movement in the Ce taceans does not vary in the different spe cies except by mo difications the im portance of which we are not able to appreciate. The dif ferences in their exterior structure, moreover, do not ap pear to exercise any influence over their mode of living; for _ _ _ the chief of these consists in the Manatee having nails to the ends of its pectoral fin, which correspond to the fingers, of which it is in part composed ; and in its tail being oval instead of being extended laterally into two wings.
We have in no way considered as forming part of the organs of movement, the protu berances which are seen upon the back of some species of spouting Cetaceans, some times in the form of a hump, and sometimes like a fin, more or less elevated. These pro tuberances, in fact, are nothing more than simple gibbosities, simple prolongations of the skin, filled with dense cellular tissue and fat, and resemblina more or less a fin, but devoid of any indepenClent movement, and without any direct connection either with the vertebrm of the back or with the muscular system.
Digestive organs. — The alimentary appa ratus is one of those, which, in many of its parts, presents the most important modifica tions in the Cetaceous Order.
The three genera into which the Herbivorous Cetaceans are divided, are characterized by three systems of dentition fundamentally dif ferent. The Manatees have molares with clou ,, n HP tritdia rirltrne nnti with thP r 5„, , root distinct from the crown, pre senting a remarkable resemblance to those of some of the Pachy derms, as the Hippopotamus. The Dugongs have simple elliptical molares, the crown of which, before it is worn, presents two slight fur rows, which are entirely effaced by age. They are without fangs, properly so called ; and in the up ner iaw are found two lonz tusks, IL of which the other Cetaceans this family are destitute. The Rytinre have no molares at all ; these teeth are replaced by a horny plate in the middle of each jaw a structure which seems to connect these anima2ls with tbe Whalebone Whales.
The tongue is short and but little susceptible of movement.
The os hyoides is characterized in the Cetacea chiefly by the slight degree or total absence of connection with the larynx, resulting from the elevated position of this organ required by its peculiar relations with the posterior nares.
In the Herbivorous order the Dugong pre sents a simple form of the os hyoides ; the posterior cornua soon anchylose with the body, but send no ligament to the thyroid cartilage. The anterior comua generally remain cartila.:. ginous, and form the medium of union be tween the body or basi-hyal, and the large and long styloid processes. In the Delphinide the body and posterior cornua of the hyoid bone are of a flattened form. In the Baleenidx, as the Piked Whale or Balzenoptera, the body (a, fig. 258) is a cylindrical bone, extended The genio-glossi pass backwards and inwards from the anterior contour of the lower ja‘v.
The tongue itself corresponds to the form of the space included by the rami of the lower jaw, and is consequently of great size in the Cachalots and Baloenid, rising in the latter like an immense cushion (a, fig. 259), into the space between the laminw of baleen (b), and affording a great quantity of the finest oil. In the figure it is represented in the Piked Whale, but probably preternaturally enlarged and raised by the extrication of gas caused by putrefaction. It is thick, and its free extremity is generally short, but this is less remarkable in the Phytophaga than in the Zoophaga. In the Dugong (fig. 260) the upper surface of the anterior part of the tongue (a) is beset with cuticular spines, and on each side of its basis there is a remarkable horny retroverted pointed process (b, b).
transversely, and is slightly curved backwards and upwards; its middle portion supports an teriorly two processes (b, b) resembling the base of the anterior cornua in the Ruminants ; besides these there are, in this genus, two rounded tubercles on the posterior margin op posite these processes. The styloid bones (c, c) are cylindrical and slightly curved in two directions ; they are joined by cartilage on each side to the occipital protuberance which represents the mastoid process.
The muscles which protrude and retract the tongue are extremely simplified in ,the Ce taceans ; the retractors are represented by a single pair, analogous to the stylo-hyoidei, the fibres of which pass from the posterior margin of the stylo-hyal bones to the body of the hyoid. The stylo-glossi pass from the anterior and superior margin of the styloid process to their insertion. The hyoglossi arise from the middle of the convexity of the os hyoides.