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, differently 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 vertebre 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 pelagic 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 (Balenide) ; 
the sperm whales (Physeterid@) ; the dolphins (Delphinide) ; and the hete- 
rodonts (Heterodontide). 
Fam. 1. Batavia, includes those gigantic marine mammals whose jaws 
are edentate or toothless. Instead of teeth, there are on the upper jaw 
horny lamine, situated transversely and parallel to each other upon two 
rows. These are the whalebones, 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 Balenoptera 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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