Whalebone Whales never possess permanent teeth as well as the baleen; but in the foetus are more than traces of true teeth, which, however, never arrive at maturity. The whalebone itself is described later (p. [354]). That the Whalebone Whales possess teeth while in the foetal condition was discovered so long ago as 1807. It has since been confirmed by many observers. Not only is there one set of teeth developed in the foetal Balaenoptera but two, of which one comes to a greater maturity; the other, in fact, remaining at a very early stage of development. The more complete dentition belongs to the milk series, as is the case with the Toothed Whales. A very interesting conclusion with regard to the derivation of the simple conical teeth of Whales seems to follow from the development of these structures in Balaenoptera. There are in the young foetus fewer teeth than in the more advanced embryo. Now in the younger embryo some of the teeth are furnished with more than one cusp; they are bi- or even tri-conodont. As Sir R. Owen observed, the teeth—some of
them—are literally double teeth. This is a suggestion of the more complicated teeth of the Zeuglodonts, and shows so far that the simple conical teeth of existing Whales (cf. however the Platanistidae) are not by any manner of means so primitive as their actual structure would undoubtedly lead one to believe. Further than this, the greater number of teeth in the older embryo coincided with the disappearance of these double teeth, which seem to split up into the simple conical teeth.
The Toothed Whales are not furnished with baleen, but with teeth only. These teeth are more or less numerous, their arrangement being of value in the classification of the group; a matter which is dealt with later.
In the Narwhal, whose dentition in the adult is reduced to the well-known tusk or tusks (properly developed only in the male), there is a complete foetal dentition. A very curious fact has been elucidated by Professor Kükenthal about the dentition of the Common Porpoise. It appears that in this Cetacean the two teeth corresponding to each other of the two dentitions may fuse into a single tooth, which has in consequence a double crown. It may be that this is the case with the Platanistid Inia, and that its diconodont teeth are not, therefore, a reminiscence of the comparatively complicated teeth of the ancient Zeuglodonts.
The internal organs of Whales which show the greatest peculiarities as compared with other mammals are the stomach, the lungs, and the diaphragm. Whales always possess a complicated stomach divided into many, but into a variable number of, chambers: there are as few as four in some, as many as fourteen in Ziphioids.
On account of its complication the stomach[[225]] has been compared to that of Ruminants—it has even been alleged that Whales "ruminate"—but the comparison will not hold good. Nor, on the other hand, is there a very close resemblance to the equally-complicated stomach of the Sirenians.
The Rorqual has a stomach with as few compartments as any. The only Whale which appears to have fewer is Balaena mysticetus, where there are but three. In the Rorqual the oesophagus opens into a more or less globular sac; from the upper end of this, i.e. close to the entry of the oesophagus, arises the second chamber, long and narrowish; then follows an extremely short third sac,
then a larger fourth, after which comes the dilated commencement of the small intestine. The latter might be regarded as a chamber of the stomach were it not for the fact that the ducts of the liver and the pancreas open into it. This represents one type of the Cetacean stomach, which seems to be found in all Whales except the Ziphioids. In the latter, the oesophagus opens into the first compartment as usual; but the second division of the stomach arises not close to the entrance of the oesophagus, but at the opposite end. It would seem, therefore, as if the first division of the stomach, found in most Whales, were missing in Ziphioids. This way of looking at the matter is confirmed by the fact that in Hyperoodon a remnant of the missing first stomach is found in the shape of a small diverticulum of the oesophagus just before it enters the stomach.
The essential difference between the Whale's and the Ruminant's stomach is this: in the latter the stomach is primarily divided into two portions, of which the first is non-digestive and is clothed with oesophageal epithelium. The second, the abomasum, is the digestive region. The first part is again divided into three compartments. In the Whales, on the other hand, it is the digestive part which is again subdivided, while if the first part is divided it is not markedly so as in the Ruminants.
The lungs are remarkable for their unlobulated character; in this they agree with the lungs of the Sirenia. The thoracic cavity in which they lie is barrel-shaped, and not, as is usual in terrestrial mammals, boat-shaped, i.e. narrower sternally than above. The alteration of the shape of the thoracic cavity is associated with the aquatic life; so at any rate the fact that it is also marked in Seals and even in the Otter seems to show. The Whales are also characterised by the great obliquity of the diaphragm, which is extremely muscular. In this character again we find an agreement with the Sirenia, and also with other aquatic mammals; it is not therefore a character of Whales so much as evidence of an adaptation to the aquatic life. The advantage is, it appears, in the increased capacity of the thoracic cavity, and the consequent greater possibilities of expansion of the lungs, which it must be remembered serve as hydrostatic as well as breathing organs.