308 MAMMALIA. 



only be moved as a whole. The hind limbs are wholly wanting as 

 external appendages. 



The skull, as compared with the large facial part of the head, 

 which is often elongated like a beak, is small and often asymmetrical, 

 the right side being the largest. Its bones are separated by 

 sutures and loosely connected. The parietals early fuse with the 

 interparietals to a single bone. The hard petrous bone remains 

 isolated from the other parts of the temporal bone. The nasal 

 cavity, in connection with the great development of the premax- 

 illaries, is shifted entirely on to the skull. Except in the Sirenia, 

 the nasal bones are rudimentary. The jaws are frequently alto- 

 gether without teeth. A change of dentition takes place only in 

 the Sirenia. In the true Cetacea the dental germs are developed in 

 foetal life; but the teeth either fall out before birth (Whalebone 

 Whales), or develope into the permanent teeth (Dolphins). Of 

 the hind limbs traces are only sometimes found, as small bones which 

 are interpreted as the rudiment of a pelvis ; in Balcena mysticetus 

 rudiments of a femur and tibia are also present (fig. 688). The 

 single or double nasal aperture is placed more or less high up on the 

 skull, and leads straight down into the nasal cavities, which descend 

 as a paired, but posteriorly single, nasal canal, which at the soft palate 

 can be shut off from the pharynx by a sphincter muscle. The view 

 that the Whale spouts water through its nasal apertures has been 

 proved to be erroneous. The expired aqueous vapour condenses 

 into a cloud, and gave rise to the illusion that a column of water 

 was ejected from the nostrils. The lungs are very spacious ; they 

 extend, like the swimming bladders of Fishes, far backward, and play 

 an essential part in the maintainance of the horizontal position 

 of the body in water; the diaphragm also has a corresponding 

 horizontal position. Saccular dilatations on the aorta and pulmonary 

 arteries, as well as the so-called arterial networks may serve as aids 

 to respiration during diving. 



The females bear a single (the smaller species rarely two) 

 relatively far advanced young, which, however, need the care of the 

 mother for a long time after birth. The two teats of the mammae 

 lie in the inguinal region, in the Sirenia on the thorax. 



The Whales usually live together in herds. The smaller species 

 frequent the coasts, and even enter the mouths of rivers. The 

 larger species prefer the open sea and colder climates. They swim 

 with great strength and speed, usually keeping near the surface. 

 The gigantic whalebone Whales, which are entirely without teeth, 



