Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/415

Rh CETACEA.] MAMMALIA water. It also often happens, especially when the surface of the ocean is agitated into waves, that the animal com mences its expiratory puff before the orifice has quite cleared the top of the water, some of which may thus be driven upwards with the blast, tending to com plete the illusion. In hunting Whales the harpoon often pierces the lungs or air passages of the unfortunate victim, and then fountains of blood may be forced high in the air through the blowholes, as commonly depicted in scenes of Arctic adventure ; but this is nothing more (allowance being made for the Whale s peculiar mode of breathing) than what always follows severe wounds of the respiratory organs of other mammals. All the Cetacea are predaceous, subsisting on living animal food of some kind. One genus alone (Oreo) eats other warm-blooded animals, as seals, and even members of its own order, both large and small. Many feed on fish, others on small floating crustaceans, pteropods, and medusae, while the principal staple of the food of many is constituted by the various species of cephalopods, Loligo and other Teuthidce, which must abound in some seas in vast numbers, as they form almost the entire support of some of the largest members of the order. In size the Cetacea vary much, some of the smaller Dolphins scarcely exceeding 4 feet in length, while others are the most colossal of all animals. It is true that most statements of their bulk found in general and even zoological literature are greatly exaggerated, but even when reduced to their actual dimensions (which will be mentioned under the respective genera) some of the existing Whales exceed in size that of any animal living either at present or in former times of which we have any certain evidence. With some excep tions, the Cetacea generally are timid inoffensive animals, active in their movements, and very affectionate in their disposition towards one another, especially the mother towards the young, of which there is usually but one, or at most two, at a time. They are generally gregarious, swimming in herds or &quot; schools &quot; (so termed by the whalers) sometimes amounting to many thousands in number ; though some species have hitherto only been met with either singly or in pairs. Relations of the different Cetacea to each other and to other Mammals. As before said, the Cetacea form a per fectly well-defined group, sharply separated from all other mammals, and with no outlying or doubtful forms at present known. Among the existing members of the order, there are two very distinct types, the Toothed Whales or Odontoceti and the Baleen Whales or Mystacoceti, which present as many marked distinguishing structural characters as are found between many other divisions of the Mammalia which are reckoned as orders. The extinct Zetiglodon, so far as its characters are known, does not fall into either of these groups, but is in some respects an annectant form, and therefore must be placed, provi sionally at least, in a third group by itself. The Mystacocetes appear at first sight to be the most specialized and aberrant of the existing Cetacea, as indicated by the absence of teeth, the presence of baleen, and the form and size of the mouth ; but, as we see in other groups, dental characters, and all such as relate to the prehension of food generally, are essentially adaptive and consequently plastic or prone to variation, and hence cannot well be relied upon as tests of affinity. In another character, also adaptive, the laxity of the connexion of the ribs with the vertebral column and with the sternum, and the reduction of that bone in size, -allowing great freedom of expansion of the thoracic cavity for prolonged immersion beneath the water, the Mystacocetes have passed beyond the Odonto- cetes in specialization. On the other hand, the greater symmetry of the skull, the more anterior position of the external nostrils and their double external orifice, the form of the nasal bones, the presence of a distinctly developed olfactory organ, the mode of attachment of the periotic bone to the cranium, the presence of a caecum and the regular arrangement of the alimentary canal, the more normal characters of the manus and the better development of the muscles attached to it, and the presence, in many species at least, of parts representing not only the bones but the muscles and ligaments of a hind limb, 1 all show less deviation from the ordinary mammalian type than is presented by the Odontocetes. Taking all these characters into consideration, it does not appear reasonable to suppose that either type has been derived from the other, at all events in the form in which we see it now, but rather that they are parallel groups, both modified in different fashions from common ancestors. Among the Mystacocetes, in the especially distinguish ing characters of the division, the Balxnopttrse are less specialized than the Bal&nsR, which in the greater size of the head, the length and compression of the rostrum, the development of the baleen, and shortness of the cervical region are exaggerated forms of the type, and yet they retain more fully some primitive characters, as the better development of the hind limb, the pentadactylous manus, and the absence of* a dorsal fin. Both forms are found distinct in a fossil state as far back as the early Pliocene age, but generally represented by smaller species than those now existing. The Mystacocetes of the Miocene seas were, so fur as we know at present, only Balxnopterx, some of which (Cetotheriuni) were, in the elongated flattened form of the nasal bones, the greater distance between the occipital and frontal bone at the top of the head, and the greater length of the cervical vertebrae, more generalized than those now existing. In the shape of the mandible also, Van Beneden, to whose researches we are chiefly indebted for a knowledge of these forms, discerns some approximation to the Odontocetes. Among the last-named group there are several distinct types, of which that represented by Platanista, although in some respects singularly modified, has been considered to present on the whole approximations towards the more normal and general type of mammalian structure. It is therefore interesting to find a similar form well represented among the earliest fossil remains of Cetaceans in Europe. Almost all the other members of the suborder range them selves under the two principal heads of Ziphioids (or Physeteroids) and Delphinoids. The former is an ancient and once abounding type, of which the Sperm Whale (Physeter) is a highly specialized form. Among the latter, Globiceplialus is a modified form as regards the structure of its anterior extremity, and Monodon as regards its dentition, while Delphinus with its various minor subdivi sions may be regarded as the dominating type of Cetaceans at the present day, abundant in slightly differentiated species and abundant in individuals. They are in this respect to the rest of the order much as the hollow-horned Ruminants are to the Ungulates. The earliest Cetaceans of whose organization we have anything like complete evidence are the Zeuglodons of the Eocene period, 2 which approach in the structure of skull and teeth to a more generalized mammalian type than either of the existing suborders. The srnallness of the cerebral cavity compared with the jaws and the rest of the skull they share with the primitive forms of many other types. The forward 1 These have recently been described in detail by Professor Struthers in the Journal of Anatomy and Physiology, 1881. 2 The cervical vertebra of Palseoceius, supposed to be from the Cam bridge Grernsand, and a single caudal vertebra lately found in the Upper Eocene at Roydon in Hampshire, may for the present be omitted from consideration, as too inconclusive in the nature of the evidence they afford as to the history of the group. XV. 5