30 



THE ANATOMY OF VERTEERATED ANIMALS. 



a 



chial clefts ; within this is developed a series of raylike mem- 

 brane bones, termed opercular and brcmchiostegal, wliicli be- 

 come closely connected ^Yitll the hyoidean arch. A corre- 

 sponding process of the skin is developed in the Batrachian 

 Tadpole, and gTows backward over the branchia3. Its posterior 

 edge, at first free, eventually unites with the integument of 

 the body, behind the branchial clefts, the union being com* 

 pleted much earlier on the right side than on the left. 



In most mammals a similar fold of integument gives rise 

 to the pinna, or external ear. 



The branchial skeleton bears 

 the same relation to the posterior 

 visceral arches that the hyoidean 

 does to the second. When fully 

 developed, it exhibits ossified lat- 

 eral arches, connected by median 

 pieces, and, frequently, provided 

 with radiating appendages which 

 give support to the branchial mu- 

 cous membrane. It is only found 

 in those Vertehrata whicb breathe 

 by gills — the classes I^isces and 

 Amphibia. In the higher Verte- 

 brata, the posterior of the two 

 pairs of cornua, with which the hy- 

 oidean apparatus is generally pro- 

 vided, are the only remains of the 

 branchial skeleton. 



The skull and face are usually 

 symmetrical in reference to a me- 

 dian vertical plane. But, in some 

 ^etacea^ the bones about the re- 

 gion of the nose are unequally 

 ^developed, and the skull becomes 

 asymmetrical. In the Flatfishes 

 {Pleuronectidoi)^ the skull be- 

 ^ comes so completely distorted, that 



FiG.lO.— ThesTviinofariaice(PZa/t-.«.<(a ,-. x ^ ^ ^ 1- 1 ^ •\ £ 



Tulf,ariH\ viewed froin above. The tlie twO CyCS lie On OUe Siclc Ot 



dotted line «, 6 is the true morpho- x\\Q bodv, which is, in some cases, 



losical median hne; 6>r, c'/-, the po- ^ c " -\ • \^ ji 'i/ 



Biiionof the twoeyes in their orbits; the Icit, and, in OtllCrS, the right 



side. In certain of these fishes, 

 the rest of the skull and facial 

 bones, the spine, and even the 

 limbs, partake in this asymmetry. The base of the skull and 



Eth, ethmoid ; Prf^ prefrontal ; Fr^ 

 left frontal ; Fr\. rifrht fiontal ; Pa, 

 parietal; >S'6>, supra-occipital; Ep.O^ 

 epiotic. 



