SCYLLIUM 



49 



Meckel's cartilage and ceratohyal the 

 There is no difficulty in recognising the fact that the jaws 

 are simply slightly modified visceral arches. These carti- 

 laginous arches lie in the splanchnopleur. Stiffening the 

 partitions between the gill-slits are extrabranchials and 

 branchial rays. 



It is important to notice that these jaws and branchial 

 arches, which together constitute the splanchnocranium, are 

 not fused on to the neurocranium or attached to it otherwise 

 than by ligaments. 



The pterygo-quadrate is slung from the skull by the 

 hyomandibula, the upper end of which is attached to the 

 auditory capsule. This method of suspension of the upper jaw 

 is called hyostylic. The upper jaw does not touch the neuro- 

 cranium itself. In addition to its attachment by the hyomandi- 

 bula, there are two ligaments, the ethmoid and the post- 

 spiracular, which tie the upper jaw to the brain-case. 



The ordinal numbers by which the arches, segments, and 

 slits are known are unfortunately liable to lead to confusion, 

 for which reason they are tabulated below : 



Slits or Clefts. 



i st visceral = spiracle 

 2nd visceral = i st gill slit 

 3rd visceral = 2nd gill slit 

 4th visceral =3rd gill slit 

 5th visceral =4th gill slit 

 6th visceral = 5th gill slit 



Vertebral Column. — Corresponding to each septum between 

 two segments, there are paired basidorsal and basiventral 

 cartilages, surrounding the notochord. The sheath of the 

 notochord is penetrated by these cartilages which, together, 

 form a bobbin-like ring or centrum, which constricts and 

 interrupts the notochord. The centra articulate on one 

 another end to end, and in this way a vertebral column is 



