128 



THE ORIGIN OF VERTEBRATES 



bars of other higher fishes, in that it forms a system of cartilages 

 which lie external to the branchia) — an extra-branchial system. 



This branchial basket-work is simpler in Ammoccetes than in 

 Petromyzon, and its actual starting-point consists of a main trans- 

 verse bar corresponding to each branchial 

 segment ; from this transverse bar the 

 system of longitudinal bars by which 

 the basket-work is formed has sprung. 

 These transverse bars arise from a 

 cartilaginous longitudinal rod, situated 

 close against the notochord on each 

 side. These rods may be called the 

 subchordal cartilaginous bands (Fig. 53), 

 and, according to the observations .of 

 Schneider and others, each subchordal 

 band does not form at first a continuous 

 cartilaginous rod, but the cartilage is 

 conspicuous only at the places where 

 the transverse bars arise. In the 

 youngest Ammoccetes examined by 

 Schaffer, he could find no absolute dis- 

 continuity of the cartilage except be- 

 tween the first two transverse bars, but 

 he says that the thinning between the 

 transverse bars was so marked as to 

 make it highly probable that at an 

 earlier stage there was discontinuity. 

 The whole system of branchial bars and 

 subchordal rods is at first absolutely 

 disconnected from the cranial system of 

 trabecule and parachordals, and only 

 later do the two systems join. 



These observations on Ammoccetes 

 lead most definitely to the conclusion 

 that the starting-point of the whole cartilaginous skeleton of 

 the vertebrate consisted of a series of transverse cartilaginous bars, 

 for the purpose of supporting branchial segments ; these were con- 

 nected with two axial longitudinal cartilaginous rods, which at first 

 contained cartilage only near the places of junction of the branchial 



Fig. 54. — Ventral View of 

 Head Region of Ammoccetes. 



Th., thyroid gland; M., lower 

 lip, with its muscles. 



