THE EVIDENCE OF THE SKELETON 127 
of hard cartilage, the only soft cartilage in this region being that 
which forms the nasal capsule, not represented in Fig. 53, B. 
These two groups of cartilage arise independently, so that at first 
the basi-cranial system is quite separate from the branchial, and only 
late in the history of the animal is a junction effected between the 
branchial system and the trabecule and parachordals, an initial 
separation which is especially striking when we consider that in this 
animal all the cartilaginous structures of any one system are con- 
tinuous: there is no sign of anything in the nature of joints. 
Of these two main groups, the branchial cartilages are formed first 
in the embryo, a fact which suggests that they are the most primi- 
tive of the vertebrate cartilages, and that, therefore, the first true 
formation of cartilage in the invertebrate ancestor may be looked for in 
the shape of bars supporting the branchial mechanism. The evidence 
of the origin of the cartilaginous structures in Ammocostes is given 
by Shipley in the following words :— 
“The branchial bases are the first part of the skeleton to appear. 
They arise about the 24th day as straight bars of cartilage, lying 
external and slightly posterior to the branchial vessel. 
“ The first traces of the basi-cranial skeleton appear on the 30th 
day as two rods of cartilage—the trabecule.” 
Our attention must, in the first place, be directed to this branchial 
basket-work of Ammoccetes. 
Underlying the skin of Ammoccetes in the branchial region is 
situated the sheet of longitudinal body-muscles, divided into a series 
of segments or myotomes, which forms the somatic muscles so cha- 
racteristic of all fishes. This muscular sheet is depicted on the left- 
hand side of Fig. 54. It does not extend over the lower lip or over 
that part in the middle line where the thyroid gland is situated. In 
these parts a sheet of peculiar tissue known by the name of muco- 
cartilage lies immediately under the skin, covering over the thyroid 
gland and lower lip. The somatic muscular sheet with the super- 
jacent skin can be stripped off very easily owing to the vascularity 
and looseness of the tissue immediately underlying it. When this is 
done the branchial basket-work comes beautifully into view as is 
seen on the right-hand side of Fig. 54. It forms a cage within which 
the branchie and their muscles lie entirely concealed. 
This is the great characteristic of this most primitive form of the 
branchial cartilaginous bars and distinguishes it from the branchial 
