182 ZOOLOGY. 



arch.) But what has become of the hyomandibnlar in 

 higher types ? Considering the articulation of the hyoman- 

 dibnlar with the otic capsule, and its position immediately 

 behind the spiracle (= ear-drum), it is natural to suppose 

 it may be represented by the columetta of the frog and 

 stapes of the rabbit. But the development of the hyoid 

 arch in these higher types is not yet made clear enough to 

 definitely establish the homology. 



It is important to realize that the cartilaginous branchial 

 arches lie internal to (i.e. nearer the cavity of the pharynx 

 than) the afferent branchial arteries : the efferent ones 

 lie alongside them. The basi- branchial similarly lies nearer 

 the pharynx-cavity than the truncus arteriosus; it even 

 extends back into the region of the heart and lies on the 

 dorsal side of the pericardial cavity. This is an important 

 point, for it enables us to plainly distinguish visceral arches 

 from such cartilages (or bones) as ribs and limb-girdles. 

 The latter are part of the body-wall : they are formed from 

 somatic mesoblast, as we shall learn to call it shortly. But 

 the visceral arches belong to the wall of the alimentary 

 canal (splanchnic mesoblast). Owing to the obliteration 

 of the coalom (which separates body-wall from alimentary 

 canal) in the throat region, this difference is easily over- 

 looked, and at one time the limb-girdles were regarded as 

 modified branchial arches. 



Besides the main cartilages already mentioned, there are, 

 in the head and throat, certain other much more superficial 

 ones. Outside the olfactory capsule there are two pairs of 

 rod-like rostral cartilages ; outside the jaw-cartilages are 

 two pairs of labials (fig. 93) ; and outside the cerato- 

 branchials a series of extra-branchials. It should also be 

 mentioned that the hyomandibular and cerato-hyal, and 

 the first four epi- and cerato-branchials, carry cartilaginous 

 gill-rays to support the gill-processes. 



13. Fins and Fin-girdles (figs. 95 and 96). The 

 paired fins and their girdles differ considerably from the 

 limbs and limb-girdles of frog and rabbit, yet there is 

 enough resemblance to prove their homology. At the same 

 time there is a remarkable resemblance between the skeleton 



