TELEOLOGY OF THE SKELETON OF FISHES. 153 



cation of the branchial arches in connection with the transitory 

 manifestation of cartilaginous branchial arches in the larvae of the 

 Batrachia : this is one of tliose similarities that has led to the meta- 

 phorical expressions of " gigantic tadpoles," as applied to fishes. 

 But we see how admirably the branchial arches are adapted to the 

 aquatic respiration of the fish, by their advance to a grade of deve- 

 lopment, Avhich they are never destined to attain in the frog. 

 Observe their firm ossification, their elastic joints, the sieve-like 

 valves developed from the side next to the mouth, pre-arranged with 

 the utmost complexity and nicety of adjustment to prevent the entry 

 of any particles of food or other irritating matters into tlie inter- 

 space of the tender, highly vascular, and sensitive gills. Observe, 

 also, how the last pair of these arches is reduced to the capacity of 

 the pharynx which it surrounds, how it is thickened in order to 

 support teeth of multiform character, according to the nature of the 

 food ; in short, converted into an accessory pair of jaws, and the 

 most important of the two. In no other Vertebrate Animals is the 

 mouth provided with maxillary instruments at both fore and hind 

 apertures : in no other part of the ichthyic organisation is the special 

 divergence from any conceivable progressive scale of ascending 

 structure culminating in Man so plainly mai'ked as in tliis. 



All writers on Animal Mechanics have shown how^ admirably the 

 whole form of the fish is adapted to the element in which it lives 

 and moves : the viscera are packed in a small compass, in a cavity 

 brought forwaixls close to the head ; and w^hilst the consequent abro- 

 gation of tlie neck gives the advantage of a more fixed and resisting 

 connection of the head to tlie trunk, a greater proportion of the trunk 

 behind is left free for the development and allocation of the muscular 

 masses which are to move the taih In the caudal, which is usually 

 the longest, portion of the trunk, transverse processes cease to be 

 developed, whilst dermal and intercalary spines shoot out from the 

 middle line above and below, and give the vertically extended, com- 

 pressed form, most efficient for tlie lateral strokes, by the rapid alter- 

 nation of which the fish is })ropelled forwards in the diagonal, be- 

 tween the direction of those forces. The advantage of the bi-concave 

 form of vertebra with intervening elastic cajisules of gelatinous fluid, 

 in effecting a combination of the resilient with the muscular power, 

 is still more obvious in the Bony Fishes than in the Shark. 



You may be reminded that all the vertebras of the trunk are dis- 

 tinct from one another at one stage of the quadruped's development, 

 as in the fish throughout life ; and you might suppose that the 

 absence of that develoi)ment and confluence of certain vertebrai 

 near the tail, to form a sacruii), was a mark of inferiority in fishes. 



