560 



CETACEA. 



deposits. The earliest known form is Zeuglodon from the 

 Eocene. 



Sub-order 1. MYSTACOCETI (BALAENOIDEA.) 



The whalebone whales, with large head, without teeth in the 

 jaws, with whalebone (baleen) hanging from the palate. The 

 oesophagus is narrow, there are two spiracles (external narial 

 openings), and the nasal passages are without saccular dilata- 

 tions. Manus tetradactyle except in Balaena. 



Fam. 1. Balaenidae. Teeth are absent in the adult state, but numer- 

 ous minute* calcified teeth are present in both jaws in the foetus. The 

 whalebone or baleen consists of a number of horny triangular plates 

 which are arranged in a row on each side of the palate and hang down 

 into the mouth. The outer edge of the plate is smooth, its base is attached 

 to a transverse ridge of the palate, and its inner edge is frayed out into 



Fig. 288. — Skull of Balaena mysticetus, with the whalebone (R6gne animal). 



numerous filamentous processes. A vascular lamella extends from the 

 palate into the base of each plate. From this there projects a number 

 of vascular papillae wliich extend into the free bristle-Uke fibres. The 

 epithelium romid these papillae is cornified, and in the proximal region 

 gives rise to a connecting substance (" enamel") which forms the smooth 

 outer substance of the plate, and with the contained fibres constitutes 

 the plate. Peripherally tliis connecting substance breaks down and the 

 fibres become free. The " gum " (intermediate substance) is a mass of 

 thickened epithelium between the bases of the plates. In feeding, the 

 animal opens its mouth so that the baleen plates hang vertically down- 

 wards. In Balaena, in which the plates are very long, their lower ends rest 

 on the base of the mouth within the lower Ups wliich prevent their bending 

 outwards when the mouth is closed. The animal then closes its mouth 

 and the water is driven out through the strainer formed by the plates 

 and their fibres. It then swallows by the help of its tongue the organ- 

 isms which are retained by the strainer. The ends of the plates and their 

 fibres fold backwards when the mouth is closed. 



The skull is symmetrical, and the nasal bones are larger than in other 



