SKELETON 



69 



In the more primitive vertebrates the trough is converted into a 

 tube around the brain by the extension of cartilages between the ali- 

 sphenoid cartilages and the otic capsules of the two sides dorsal to the 

 brain. This roof or tegmen cranii is usually incomplete, having one 

 or more gaps or fontanelles, closed only by membrane. In the higher 

 vertebrates the cartilage roof is at most restricted to a mere arch, the 

 synotic tectum, between the otic capsules of the two sides. 



Later a pair of nasal capsules de- 

 velop around the olfactory organs. 

 These are usually fenestrated and be- 

 come united to the cornua, alisphe- 

 noids, and ethmoid plate. In a similar 

 way a sclera (sclerotic coat) forms 

 around each eye, but since the eye 

 must move, this sense capsule never 

 unites with the rest of the cranium. 

 Behind the otic capsules a varying 

 number of (four in some sharks and 

 most teleosts, in others three, in am- 

 phibia two, fig. 91) occipital vertebrae 

 are developed, which later fuse with 

 the rest of the chondrocranium. They 

 alternate with myotomes and nerves in 

 this region as do the vertebrae of the 

 vertebral column. 



The cartilaginous visceral skeleton 



arises in the pharyngeal region, which chondrocranium of an eksmobranch, 

 , , , 1 ,1 .,, straightened out. Compare with 



is weakened by the presence of the gill still earlier fig. 67. als, alisphenoid; 

 clefts. It consists of a series of pairs ^[■J°7,";,;7eSr'h?pWt"e°os 



of bars, the visceral arches (fig. 69, oc, otic capsule; ov, occipital verte- 

 T izTT\ -I • • ^1 ...I. ... ...1 brae; n, notochord; pc, parachordal 



I -VII), lying in the septa between the piate; tr, trabeculie. 

 clefts, the bars of a pair being con- 

 nected below the pharynx. Each bar, at first, is a continuous struc- 

 ture, but to allow of changes of size in the pharynx, each becomes 

 divided into separate parts, while the arches become connected in the 

 mid-ventral line by unpaired elements, the copulse. The two anterior 

 arches are specialized and have received special names, the first being 

 the mandibular, the second thehyoid arch, the others, in the region of 

 the functional gills, being called collectively gill or branchial arches. 

 The number of these last varies with the number of gill clefts, there 



(platybasic) 



Fig. 68.— Early 



