These are six in number, three being dorsal, one caudal and two anal.
The dorsal and anal fins each consist of two sets of structures, the fin-rays and the interspinous bones. Each fin-ray forms a delicate, nearly straight, bony rod which becomes thickened and bifurcated at its proximal or vertebral end, while distally it is transversely jointed and flexible, frequently also becoming more or less flattened.
The first dorsal fin has thirteen rays, the second, sixteen to nineteen, the third, seventeen to nineteen. The first anal fin has about twenty-two, the second anal fourteen. In each fin the posterior rays rapidly decrease in size when followed back.
The interspinous bones of the dorsal and anal fins alternate with the neural and haemal spines respectively, and form short, forwardly-projecting bones, each attached proximally to the base of the corresponding fin-ray.
The caudal fin consists of a series of about forty-three rays which radiate from the posterior end of the vertebral column, being connected with the urostyle or hypural bone, and with the posterior neural and haemal spines without the intervention of interspinous bones. Like the other fin-rays those forming the caudal fin are transversely jointed, and are widened and frayed out distally.
The tail-fin in the Cod is homocercal, i.e. it appears to be symmetrically developed round the posterior end of the vertebral column, though in reality a much greater proportion is attached below the end of the vertebral column than above it. It is a masked heterocercal tail.
The Skull.
Owing to the fact that very little cartilage remains in the skull of the adult Codfish, its relation to the completely cartilaginous skull of the Dogfish is not easily seen. Before describing it therefore, the skull of the Salmon will be described, as it forms an intermediate type.
THE SKULL OF THE SALMON[36].
The Salmon's skull consists of (1) the chondrocranium, which remains partly cartilaginous and is partly converted into cartilage bone, especially in the occipital region, (2) a large series of plate-like membrane bones.