6. auditory capsule.
In Myxinoids and larval lampreys, the notochord is enclosed in a thick chordal sheath, in connection with which in the tail region there occur cartilaginous pieces forming neural arch elements. In the trunk region, however, no cartilage occurs in connection with the spinal column, the only cartilage present being that forming the radiale of the dorsal fin. On the other hand in most species of lamprey (Petromyzon) cartilaginous pieces forming imperfect neural arches (fig. 4, B, 13) are found lying in the tough skeletogenous layer dorsal to the notochord, and extending throughout the whole length of the trunk and tail. Two of these pieces, which are probably homologous with the neural plates (see p. 72) of Elasmobranchs, occur to each neuromere, or segment as determined by the spinal nerves. The dorsal and caudal fins are supported by paired cartilaginous radiale which are connected proximally with the skeletogenous layer.
The Skull.
In Myxinoids the cranium is a mere cartilaginous floor without side walls or roof, and the trabeculae[27] end without growing forwards into cornua. In Lampreys the trabeculae grow forwards and send up plates of cartilage which meet above (fig. 4, 7) and form side walls and a roof for part of the brain case. In Lampreys a labial suctorial apparatus is well developed, including a large ring-like piece of cartilage (fig. 4, 2) which supports the oral funnel and bears a large armament of horny teeth. In Myxinoids on the other hand the labial skeleton is small and consists merely of barbels round the mouth.
The olfactory organ of Myxinoids has a very curious skeleton. It is covered with a kind of grating of cartilage which is prolonged in front into a tube composed of a series of imperfect cartilaginous rings. In Lampreys the olfactory organ opens merely by a short membranous passage. In correlation with the small development of the labial suctorial apparatus in Myxinoids the lingual apparatus is very greatly developed. The tongue in Myxine has been said to 'dominate the whole body' (Parker). It is supported by a great median cartilaginous bar which when followed forwards first becomes bifid and still further forwards becomes four-cleft.
The horny teeth in Myxinoids are chiefly borne on the very large supralingual apparatus. They form a double series arranged in the form of an arch. In Myxine there are seven large teeth and nine small ones on each side. In Bdellostoma the teeth of the two rows are more equal in size. In Bdellostoma and Myxine it has been shown that imperfect calcified teeth occur below the horny teeth.
In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed, but not excessively so. It consists of a long median cartilaginous bar which ends in front with a semicircular piece of cartilage supporting the median part of the tongue.
In both Myxinoids and Lampreys there is a complicated branchial basket apparatus, but while in Myxinoids the basket apparatus is interbranchial, formed deep within the head near the hypoblastic lining of the throat, in Lampreys it is extra-branchial and formed outside the head cavities (fig. 4, 10). The two sides of the basket apparatus in Myxine are not symmetrical. In the interbranchial basket apparatus of Myxinoids the hyoid and first and second branchial arches can be recognised. Traces of the interbranchial skeleton of Myxinoids can be detected in Lampreys, and similarly in Myxinoids, there are indications of the extra-branchial skeleton of Petromyzon. The branchial basket in Lampreys forms at its posterior end a kind of cup which supports the pericardium (fig. 4, 11).
A remarkable Cyclostome named Palaeospondylus[28] has recently been described from the Scottish Old Red Sandstone. It differs however from all living Cyclostomes, in having a spinal column formed of distinct vertebrae with well-developed neural arches. The caudal fin is well developed and the dorsal radiale are forked as in lampreys. The skull is well calcified and the auditory capsules are specially large. The mouth is very similar to that of lampreys, being circular and without jaws; it is provided with barbels or cirri. There is no trace of limbs and the average length is only about 1–1½ inches.
CHAPTER V.
(II.) GNATHOSTOMATA.
The mouth is supported by definite jaws.
ICHTHYOPSIDA.
The epiblastic exoskeleton is generally unimportant, the mesoblastic exoskeleton is usually well developed.
The notochord with its membranous sheath (1) may remain unmodified, or (2) may be replaced by bone or cartilage derived from the skeletogenous layer, or (3) may be calcified to a varying extent.
The first vertebra is not homologous throughout the whole series and so is not strictly comparable to the atlas of Sauropsids and Mammals.
The centra of the vertebrae have no epiphyses. The skull may be (a) incomplete and membranous, or (b) more or less cartilaginous, or (c) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (a) cartilaginous, (b) partially ossified, or (c) membrane bones may be developed in connection with it—if so, there is usually more than one membrane bone developed in connection with each half.
There are at least four pairs of branchial arches present during development. The sternum, if present, is not costal in origin.
Class I. Pisces.
The exoskeleton is in the form of scales, which may be entirely mesoblastic or dermal in origin (e.g. cycloid and ctenoid scales), or may be formed of both mesoblast and epiblast (e.g. placoid and ganoid scales). Large bony plates may be derived from both these types of scale. In general fish with a greatly developed dermal armour have the endoskeleton poorly developed; and the converse also holds good.
The integument of the dorsal and ventral surfaces is commonly prolonged into longitudinal unpaired fins, supported by an internal skeleton. These fins are distinguished according to their position as dorsal, caudal and anal fins. The dorsal and anal fins are used chiefly as directing organs, the caudal fin is however a most important organ of propulsion.
Three types of tail are found in fishes, viz.:—
1. The diphycercal, in which the axis is straight and the tail is one-bladed and symmetrical, an equal proportion of radiale[29] being attached to the upper and lower surfaces of the axis.
2. The heterocercal, in which the tail is asymmetrical and the axis is bent upwards, the proportion of radiale or of fin-rays attached to its upper surface being much smaller than that attached to its lower surface.
3. The homocercal, in which the tail though externally symmetrical, so far resembling the diphycercal type, is internally really heterocercal, the great majority of the radiale or of the fin-rays being attached to the lower surface of the axis.
The cranium in the simplest cases (e.g. Selachii) forms a cartilaginous box enclosing the brain and sense organs; in bony fishes it is greatly complicated. When palatine or pterygoid bones are present they are formed by the ossification of cartilage; in Sauropsida and Mammalia they are laid down as membrane bones. There is no tympanic cavity or auditory ossicle in relation to the ear.
There are two principal types of suspensorium by means of which the jaws are attached to the cranium:—
(1) The Autostylic. This is the primitive condition in which the mandibular arch articulates with the base of the cranium in front of the hyoid and in a similar manner.