The branchial arches are five in number and consist of the following parts on each side. The dorsal end is formed of the supra-pharyngeal bone, a large irregular bone covered ventrally with teeth of a fair size, and representing the fused pharyngo-branchials of the four anterior arches. Its external surface is continuous with four small epi-branchials which pass horizontally backwards and outwards. Their distal ends meet four long cerato-branchials which are directed forwards and inwards and form the principal part of the arches.
Each of the first three cerato-branchials articulates ventrally with a hypo-branchial, and the hypo-branchials of the two sides are united in the middle line by an unpaired basibranchial. The third hypo-branchial is much flattened. The fourth cerato-branchial is united by cartilage with the posterior surface of the third hypo-branchial, which it meets near the middle line.
The fifth arch consists only of the cerato-branchial, a wide structure covered with teeth and generally called the inferior pharyngeal bone.
The skeleton of the operculum consists of the same four bones as in the Salmon, namely the opercular, the infra-opercular, the pre-opercular and the sub-opercular. Of these the anterior bone, the pre-opercular, is the largest, while the infra-opercular is the smallest. The opercular has a facet for articulation with the hyomandibular.
2. The Appendicular Skeleton.
The Pectoral girdle.
This is of a highly specialised type. Membrane bones are greatly developed, and the cartilage bones, the scapula and coracoid, are much reduced in size and importance.
Fig. 13. The right half of the pectoral girdle and right
pectoral fin of a Cod (Gadus morrhua) × ½ (Brit. Mus.).
| 1. post-temporal. | 5. scapula. |
| 2. supra-clavicle. | 6. post-clavicle. |
| 3. clavicle. | 7. brachial ossicles. |
| 4. coracoid. | 8. dermal fin-rays. |