AIJTATOMY OF VERTEBRATES. B 



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

 framework supporting it relates chiefly to the mechanism of swal- 

 lowing and l^reathing, and is suspended to a pedicle common to it 

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

 sign ; but the essential internal part or ' lal_)yrinth ' is pjresent, 

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

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

 a proper chamber, hut 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 simjde, with the divisions 

 less clearly marked than in higher verteljrates ; the short and 

 wide gullet iDoing hardly distinguishable from the stomach. The 

 j)ancreatic function a}ipears to be performed by commonly more 

 or fewer ca;cal appendages to the duodenum. The heart consists 

 essentially of one aiu-icle 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 cireidation 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 

 chscs are, in some fishes, subcircular, fig. 8, r/ ; in others, 

 subelliptical, ib. /*, or elliptical ; comparatively large, but not 

 the largest amongst vertebrates. The primordial renal glands 

 (^corpora J-Folffianff) are persistent, and secrete the urine from 

 venous blood. Such are the leadino: anatomical characters of the 

 class Pisces — Fishes. 



§ 4. BeptiUan modification. — Llanjr 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. Wlien, 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 limlDS acquire the 

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

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

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

 100, B, D, — next, three-fingered, as in Proteus, — or foiu'-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 Keptiles {Reptilia, Cuv.). The heart has two auricles; 

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



