so 



THE ANATOMY OF VERTEBRATED ANIMALS. 



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

 brane bones, termed opercular and branchlostegal, which be 

 come closely connected witli the hyoidean arch. A corre- 

 sponding process of the skin is developed in the Batraohian 

 Tadpole, and grows backward over the branchiae. Its posterior 

 edge, at first free, eventually unites with the integum.ent 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. 



a 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 Vertebrata which breathe 

 by gills — the classes Pisces 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 

 Cetacea, the bones about the re- 

 gion of the nose are unequally 

 developed, and the skull becomes 

 asymmetrical. In the Flatfishes 

 {Pleuronectidcts), the skull be- 



Fio.in.-Thoskuiiof a*piaice(«<,te,,a °°"'^/ ^° Completely distorted, that 

 rii/niij'h). viewed from above. The the two ej'es lie On One side of 



dotted line a, Z;, is the true mornho- +Vi/i ■Kr^/-^TT ,^1,* I, • 



losical median line; Or. (;,■, Uiepo- ^"® "OGy, WtUCh IS, m SOme CaseS, 



sitionof the two eyes in their orbits; the left, and, in others; +lif> riVVit 



iSA, ethmoid: T'r/ prefrontal; /r! •, t ""' "^ Obliers, Me Tight 



left frontal; j-ri. right f:ontai; Pii, SKic. In Certain of these fishes 



parimi; SO, supra-occipital; Ep.o, the rest of the skull and facia] 



,. , ^ , . ,, . ^o"es, the spine, and even the 



hmbs, partake m this asymmetry. The base of the skull and 



