| OCR Text |
Show 1 9 0 6 . ] SELACHIAN FISHES. 7 2 5 sacs, each with a single external aperture *. Exoskeleton of dermal denticles which are structurally identical with the teeth ; no membrane-bones f . Endoskeleton cartilaginous, the cartilage often calcified. Gills supported by visceral arches and functional throughout life ; no lungs +. Median and paired fins with horny dermal rays § and with endoskeletal supports in the form of series of cartilaginous rods, the arrangement of which may be variously modified. Vertebral column comprising the notochord and its sheath, simple neural and haemal arches, and intermuscular elements, the so-called ribs; no supra-neural or infra-hsemal arches ||. In all the living forms, the males with intromittent organs, the mixopterygia, which are appendages of the pelvic fins. * The Teleostomi have two nostrils on each side, except in certain specialised Teleosts. It has been supposed that these are homologous with the external and internal nares of higher Vei'tebrates, the Dipneusti, in which the posterior nostril is included within the mouth, being cited as evidence of this, especially by those who consider them to be transitional between Fishes and Batrachians. The development of these parts in Reptilia, Aves, and Mammalia has also been considered to support this position, the internal nares being the remnants of open grooves originally connecting them with the olfactory pits. From this view I must dissent, regarding the external nares, whether single or divided into two, as homologous throughout the Fishes and the higher Vertebrates, and the internal nares as a new formation peculiar to the latter. In the Dipneusti the absence of prsemaxillaries and maxillaries permits the extension of the nasal sacs to the palate; but it appears to me that the posterior extension of the nasal sacs external to functional praemaxillaries and maxillaries, so as to carry the posterior nostril into the mouth, is quite impossible. In the Amphibia, the lowest group with true internal nares, their development supports the view that thejr are a new formation. For a general account, fwith opposite conclusions, see Balfour, ‘ Comparative Embryology,' ii. pp. 531-538. t The exoskeletal peculiarities of the Teleostomi may be summarised in the phrase " dermal ossification." On the body, where flexibility is a requirement, juxtaposed rhombic bony plates or " ganoid scales," arranged in parallel longitudinal and oblique series, are developed ; these are found in the more generalised forms, and in the more specialised ones may be variously modified or may disappear. Other parts of the fish become strengthened and protected by membrane- bones, and whether we examine the more primitive members of either the Chondrosteo-Teleostean or the Crossopterygio- Dipneust series, we find in each the same cranial roof-bones, paired parietals, frontals, nasals, post-frontals, and supratemporals, also the parasphenoid covering the basis cranii, the praemaxillaries and maxillaries (probably originally overlying labial cartilages like those of the Selachians), the dentary, angulare and splenial, sheathing the Meckelian cartilage, the circumorbitals, postorbitals and praeoperculum, the opercular and subopercular bones, protecting the branchial chamber, and finally a series of bones overlying the pectoral arch and connecting it with the cranium, post-temporal, supra-cleithrum, cleithrum, and clavicle. X The Teleostomi have typically either a lung or its homologue, the air-bladder. § The dermal rays of the Teleostomi differ from those of the Selachii in being more or less ossified; in order to retain their flexibility, they have become segmented. In the two subclasses the dermal rays occup3f exactly the same position with regard to the muscles and the endoskeletal supports, and they appear to me to be unquestionably homologous throughout both groups. For another view, see Goodrich, Quart. Journ. Micr. Sci. xlvii. (1904) p. 464. || In the Teleostomi, in addition to the neural arches which are present in the Selachii, we find a series of paired elements which are the dorsal equivalents of the ribs. In the living Chondrostei and Dipneusti these supra-neurals are attached proximally to the neural plates (basi-dorsals) on each side of the longitudinal ligament, and they meet above to form the neural spines. The ribs in the Teleostomes appear not to be homologous with those of the Selachians; in the former group they lie internal to the muscles and bound the abdominal cavity, whilst in the latter the}'' lie between the dorsal and ventral muscles, corresponding rather to the intermuscular bones of many true Fishes. That the Teleostome ribs |
