INTRODUCTION xxi 



The internal skeleton of a fish calls for little or no comment 

 here, and for details of the various bones the reader is referred 

 to a good text-book of zoology. In the cetaceans the bones 

 are spongy in texture and in form rather simplified, so that, in 

 the backbone, for instance, the elaborate articulation between 

 one bone and the next, common in ordinary land mammals, 

 tends to be lost. The skull is very specialized, as may be 

 seen in the accompanying figures (Figs. 4, 5), and the articu- 

 lation of the lower jaw with the skull is much less complicated 

 than in most land mammals. The 7 vertebrae of the neck are 

 very much compressed. They remain distinct in the more 

 primitive members of the group, but in the rest the first 2 or 

 3 may be fused, or all 7 merged into one bony unit. The 

 sacrum of land mammals, formed by the fusion of vertebrae 

 between the trunk and tail elements of the backbone, is 

 completely wanting in cetaceans ; all the vertebrae in this 

 region are separate, changing gradually in form from trunk 

 to tail. 



Of the internal organs it will be necessary to mention only 

 the air-bladder, an organ peculiar to Bony Fishes, and of 

 considerable importance in classification. When present, this 

 has the form of a long, cylindrical bag, with glistening silvery 

 walls, situated within the body-cavity and just below the 

 backbone. This bladder is filled with a mixture of gases, and 

 in the majority of fishes serves as a hydrostatic organ or float, 

 enabling its possessor to accommodate itself to the varying 

 pressure encountered at different depths. In a few forms, 

 however, it serves as an accessory breathing organ, functioning 

 exactly like a true lung. The air-bladder may be connected 

 with the gullet by a narrow pipe, or it may be entirely enclosed. 



Finally, in the possession of mammary glands and teats, by 

 means of which they can suckle their young after birth, the 

 cetaceans differ from all the fishes, in which no such organs 

 are developed. Even if the young of fishes are brought forth 

 alive, they are never suckled by the mother. The paired 

 mammary slits of the cetacean lie on either side of the genital 

 opening, and give access to little cavities in which lie the 

 retracted teats. There are two apertures on the under surface 

 of a whale, situated about two-thirds to three-quarters of the 

 body length from the snout ; the one nearer the head is the 



