420 ZOOLOGY. 



skin, under which is found a layer of fat, varying in thickness according to 

 the genera and species. The general structure of the skin is the same as in 

 other mammals, but in addition we find an apparatus of inhalation, com- 

 posed of vessels of an extreme tenuity, which anastomose together, and are 

 in direct communication with the arteries and veins. They are absorbing 

 vessels common to all animals which live permanently in water. This 

 apparatus is very conspicuous in some fishes. 



There is one pair of short limbs, the anterior, constructed for swimming. 

 The toes, being surrounded by a continuous membrane, give to them the 

 shape of a fin, and are used as such, these animals having been created to 

 live in water. The toes themselves possess a greater number of phalanges 

 than in any of the other mammals. The collar bone and hind limbs are 

 always wanting. The posterior extremity of the body terminates in a 

 broad, but horizontal, fibro-cartilaginous, fish-like tail, composed of two 

 lobes or paddles, a right and a left, difl^erently shaped according to the 

 genera and species. On the back, a fin-like organ is often but not always 

 observed ; it contains no bones, and consists merely of a fold of the skin. 

 The vertebrae of the neck are very short, and often soldered together. The 

 neck itself cannot be said properly to exist, the head being continuous with 

 the body, as is generally the case in fishes, and only indicated in some of 

 them by a slight contraction in this region of the body. The head pos- 

 sesses so little of motion that it cannot change its situation without the 

 whole body changing it at the same time. The eyes are exceedingly small; 

 the nostrils simple, with one or two openings, through which water is 

 ejected ; the external ear never exists ; the teeth vary very much in num- 

 ber, and in some genera instead of teeth we find beards, or the so-called 

 whalebones. Their food consists of fishes, Crustacea, and mollusca, but 

 never of plants ; their pelngic habits preclude a vegetable diet. The only 

 mode of progression among Cetacea is swimming, for which they are espe- 

 cially constituted. 



This order may be divided into four families : the whales (Bolainidce) ) 

 the sperm whales (Physeteridcß) ; the dolphins (Delphinidce) ; and the hete- 

 rodonts {Heterodontidat). 



Fam. 1. Bal^enid^, includes those gigantic marine mammals whose jaws 

 are edentate or toothless. Instead of teeth, there are on the upper jaw 

 horny laminae, situated transversely and parallel to each other upon two 

 rows. These are the lohalehones, provided along their inner margin with 

 numerous filaments of the same horny nature, by means of which the very 

 small animals on which the whales feed are retained in the mouth. One 

 whale yields from 700 to 1000 such bones, of which the largest is often ten 

 to thirteen feet long, and ten to twelve inches wide at the base. Two 

 genera compose this family. 



The genus Balcenoptera is characterized by an elongated head, which 

 has sometimes been compared to the head of the pike ; by the presence of 

 that expansion of the skin called dorsal fin on the posterior part of the back ; 

 and by folds or ridges on the anterior and inferior part of the body. It has 

 always been a matter of great difficulty to ascertain the number of species 

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