There are two investing bones also in Dipnoi, which ensheath the cartilage in Protopterus and Lepidosiren. The girdle is deeply imbedded in the muscles in the two last-named Fish, and the limb is articulated to its proximal, i.e. dorsal, extremity.
The pelvic cartilage is a continuous curved bar in Elasmobranehii, representing an ischio-pubic region, the part dorsal to the attachment of the limb, i. e. the iliac region, being absent or inconspicuous; but it is large in Holocephali, where the two halves of the girdle are united ventrally by ligament. Two minute cartilages are found in Polypterus: otherwise the pelvis is not represented in any Ganoid or Teleostean. A median cartilage bears the limbs in Dipnoi, and extends forwards as a pointed rod.
The fore-limb consists typically in Elasmobranehii of three basal cartilages, - pro-, meso-, and meta-pterygium, articulating each with a facet on the shoulder-girdle; of one or two outer rows of cartilaginous rods known as radialia, followed by horny fin-rays. The pro- is sometimes not differentiated from the meso-pterygium. Certain radialia intrude in Rays between the meso- and meta-pterygium, touching the shoulder-girdle. Polypterus among Ganoidei has rod-like ossified pro- and meta-pterygia and a broad meso-pterygium.- All other Ganoidei and Teleostei have a meso-pterygium generally imbedded in the base of the first fin-ray, some intrusive radialia and a meta-pterygium. But this region of the limb may be entirely absent in some Teleostei. The radialia are well-developed in chondrostean Ganoidei and Polypterus; they are usually small and nodular in Teleostei, a group in which movement of the limb takes place along the line of union between them and the more proximal parts. The fin-rays are ossified in Ganoidei and Teleostei, usually soft, sometimes entire, especially in the first.
The hind-limb in Elasmobranehii and the Holocephali contains often only one basal cartilage, a metapterygium, but a second is sometimes present in the former, and is usually termed pro-pterygium. It probably represents an undivided pro- + meso-pterygium. There are radialia and fin-rays, as in the fore-limb. The most posterior fin-ray is usually large, and connected with the cartilaginous bars of the claspers (pterygo-podia) in the male and their rudimentary representatives in the female. In all other Fish a metapterygium only is present, very broad and even secondarily segmented in chondrostean Ganoidei. The two metapterygia are also independent in them, except in Scaphirhynchus, where, as in bony Ganoidei and Teleostei, the two are united medianly with more or less completeness. The number of radialia is great in chondrostean Ganoidei. They have two rows; so too Polypterus; but other Ganoidei are reduced to a single row, usually inconspicuous and few. The fin-rays resemble those of the fore-limb. The Dipnoi differ from the preceding in having a limb composed of an axial series of cartilaginous pieces ranged end to end.
These in Ceratodus from the second onwards bear a bilateral series of jointed cartilaginous rods diminishing in length to the apex of the limb, and continued outwards into horny fin-rays. Protopterus ampkibius has a unilateral series of such rods, always very small, absent in P. annectens and Lepidosiren. Their limb is long and linear.
It may be noted that in some Elasmobranchii there are radialia, always few in number, on the posterior or inner edge of the metapterygium. They are probably secondary in origin. The ventral fins in all Fish have an abdominal position, except in some Teleostei, where they may shift forwards and become thoracic or even jugular in position
The muscles of the trunk and tail retain their segmentation. A fibrous septum in the median dorsal and ventral lines divides the musculature of one side of the body from that of the other. Another separates the dorso-from the ventro-lateral muscle-masses. The muscle fibres run longitudinally from one myocomma (septum between two muscle segments or myomeres) to the next. They are oblique in the ventro-lateral trunk muscles of Elasmobranchii and Ganoidei, and the corresponding region in Protopterus is divided into two oblique layers, as in Amphibia Urodela. The electric and pseudo-electric organs are apparently modified muscular tissue, except perhaps in Malapterurus (Siluroidei), a fish in which the organ lies beneath the skin and external to the muscles, and forms a nearly complete envelope round the trunk. The two organs in Torpedo (Batoidei) lie on either side the head, and are supplied by a nerve from the electric lobe of the medulla oblongata. In the Eel, Gymnotus, they replace the greater part of the ventro-lateral muscles of the tail, as do the pseudo-electric organs of Rays, Mormyrus, Gymnarchus among Teleostei Physostomi, and they are supplied by spinal nerves.
Each organ consists of a number of long hexagonal capsules, placed dorso-ventrally in Torpedo, antero-posteriorly in the remaining Fish, and containing each an electric plate, to one surface of which the nerves are distributed, whilst the other is in relation with a gelatinous substance. The nerves break up into fine branching primitive fibrils, and are remarkable for the extreme thickness of their sheaths. The neural aspect of the plate is negative electrically.
The brain fills the brain-case in embryonic Fish, but it finally occupies a very small part of that cavity, owing to the disproportionate growth of the cranial walls. The space left is filled by a fibrous and fatty arachnoid tissue. The cerebro-spinal membranes develope pigment cells, as in Amphibia. The nerve-centres are smaller in relation to the bulk of the body in this than in any other Vertebrate class, the ratio of brain to body being as low as I to 3000, and the spinal cord of a Sturgeon weighing 120 lbs. being no thicker than that of a Frog. The brain consists of the usual parts. The olfactory lobes are connected to the hemispheres by tracts of greater or less length in Elasmobranchii (with rare exceptions) and a few Teleostei, e. g. Cyprinoidei. They are hollow in Ganoidei. The cerebral hemispheres are the largest part of the brain in Dipnoi, and as a rule in Elasmobranchii; the corpora bigemina (optic lobes, mesencephalon) the largest in Teleostei. The hemispheres are solid in the last-named order, nearly or quite solid in Elasmobranchii, but contain large ventricles in Ganoidei and Dipnoi. In Ceratodus they have the greatest relative breadth, in Protoptems the greatest depth, in the class; and in the last-named the two lobes are separated nearly to their base.