ANATOMY OF VEKTEBRATES. 5 



The tongue, as an organ of taste, is hardly conspicuous; the 

 framework supporting it relates chiefly to the mechanism of swal- 

 lowing and breathing, and is suspended to a pedicle common to it 

 and the mandible. Of the organ of hearing there is no outward 

 sign ; but the essential internal part or ' labyrinth ' is present, 

 and its semicircular canals are, in most fishes, largely developed. 

 The labyrinth is devoid of a ' cochlea,' and is rarely provided with 

 a proper chamber, but is lodged, in common with the brain, in 

 the cranial cavity. The eyes are usually large, seldom defended 

 by eyelids, and never served by a lacrymal apparatus. The ali- 

 mentary canal is commonly short and simple, with the divisions 

 less clearly marked than in higher vertebrates ; the short and 

 wide gullet being hardly distinguishable from the stomach. The 

 pancreatic function appears to be performed by commonly more 

 or fewer crecal appendages to the duodenum. The heart consists 

 essentially of one auricle receiving the venous blood, and one 

 ventricle propelling it to the gills, or organs submitting that blood 

 in a state of minute subdivision to the action of aerated water. 

 From the gills the arterial blood is carried over the entire body 

 by vessels, the circulation being aided by the contraction of the 

 surrounding muscles. The blood is cold, or with a temperature 

 rarely above that of the surrounding medium. The coloured 

 discs are, in some fishes, subcircular, fig. 8, g; in others, 

 subelliptical, ib. h, or elliptical; comparatively large, but not 

 the largest amongst vertebrates. The primordial renal glands 

 {corpora Wolffiana) are persistent, and secrete the urine from 

 venous blood. Such are the leading anatomical characters of the 

 class Pisces — Fishes. 



§ 4. Reptilian modification. — Many fishes have a bladder of air 

 between the digestive canal and kidneys, which, in some, com- 

 municates by an air-duct with the gullet ; but its office is chiefly 

 hydrostatic. When, in the rise of structure, this air-bladder 

 begins to assume the vascular and pharyngeal relations, with the 

 form and cellular structure of lungs, the limbs acquire the 

 character of feet; at first, as in Lejyidosiren, fig. 41, 99, thread- 

 like and many-jointed — then bifurcate, or two-fingered, with the 

 ordinary elbow and wrist-joints of land-limbs (Amphiuma), fig. 

 100, b, d, — next, three-fingered, as in Proteus, — or four-fingered, 

 but reduced to the pectoral pair, as in Siren. From these gill- 

 retaining transitional forms, up to and including crocodiles, all 

 cold-blooded vertebrates, with lungs, breathing air directly, are 

 called Reptiles (Bej)tilia, Cuv.). The heart has two auricles; 

 the ventricle, in most, is imperfectly divided, and more or less of 



