In having the body covered with denticles formed of dentine the Selachians resemble the most primitive Agnatha, the Silurian and Devonian Coelolepidae (see CYCLOSTOMATA). Simi lar, but smaller denticles, may occur in the mouth and pharynx, and the true teeth are to be regarded as homologous structures. The teeth are attached to the membrane covering the jaws, which is continually moving outwards, so that the worn teeth drop off and are replaced; behind the series in use can be seen a number of series in reserve.
In the Selachians both median and paired fins are keel like outgrowths of the body strengthened by horny fin-rays ; their skeleton primarily consists of a series of cartilaginous rods seg and development the conclusion is drawn that the median and paired fins have a similar origin, and that they originated as out growths of the body. The limb-girdles, pectoral arch and pelvis are formed by fusion of the anterior basals, and sharks in which there is no pelvis (Cladoselachus) are the most primitive.
In Selachians the vertebral column con sists of the notochord and its cartilaginous sheath, and of dorsal and ventral series of paired cartilaginous plates ; the dorsal plates may meet above the spinal cord, or the arch may be completed by a median series of cartilages, but the whole is below the longi tudinal elastic ligament ; there are no paired elements meeting above the ligament and carrying a series of spines, as in the Pisces, and their ventral counterparts, the pleural ribs and haemal spines, are also absent. The cartilaginous sheath of the notochord may be calcified, and segmented into centra. Each vertebra has a neural plate, or basi-dorsal cartilage, perforated or notched behind for the exit of the ventral root of a spinal nerve, and another, inter dorsal, perforated or notched for a dorsal root. In the caudal region of the Euselachii the vertebrae are twice as numerous as the muscle segments, so that every other pair of basidorsals and interdorsals has no nerve exits; this condition, known as diplo spondyly, may be due to the need for greater flexibility of the tail.
The tail of the Chimaeroids is polyspondylic; i.e., the vertebrae are more than twice as numerous as the myotomes. In muscula ture, visceral anatomy, etc., the
Selachians have a general resem blance to the more primitive bony fishes ; thus the intestine has a spiral valve and the heart has a conus with several series of valves; the brain is characterized by the large divergent olfactory lobes.
In all living Selachians fertilization is inter nal, the males being provided with paired intromittent organs, mixopterygia, which are appendages of the pelvic fins, with a car tilaginous internal skeleton and with a groove or canal along the whole length of each, which is the duct of a glandular sac at its base. The eggs are large, heavily yolked, and enclosed in horny cases; in many sharks and rays the eggs are not laid, the cases are absorbed, and development proceeds within the body of the mother; some of these viviparous forms (e.g., Mustelus) have a yolk-sac placenta, and in certain rays (e.g., Pteroplatea) long uterine villi pass into the spiracles of the young and furnish a nutritive secretion.
The following classification is modified from that of Regan (1906).
Class Selachii Sub-class I. Pleuropterygii. Palaeozoic sharks with broad based, paired fins, the pelvics without any fusion of the basalia, the pectorals scarcely more advanced in structure. Cladoselachus, Cladodus, Sym morium, Cratoselache.
Sub-class 2. Acanthodii. Palaeozoic sharks with a strong spine at the anterior edge of each fin. As in the Pleuropterygii, the paired fins were broad-based, and mix opterygia appear not to have been de veloped. Acanthodes, etc.
Sub-class 3. Ichthyotomi. Palaeozoic sharks in which the pectoral basalia form the segmented axis of a biserial fin like that of Ceratodus (lung fish) ; males with mixopterygia. Pleuracanthus.
Sub-class 4. Euselachii (modern sharks and rays). Pectoral fin with metaptery gium (fused basalia), and with proptery gium and mesopterygium, formed by union of proximal segments of anterior radials ; pelvic fins with pelvis and basipterygium ; males with mixopterygia. Pterygo quadrate free from skull. Hyomandibular articulating with skull and supporting jaws. Gill-clefts with separate external openings.
Pectoral fins free from head; gill-openings lateral. Hyomandi bular with cartilaginous rays supporting the first half-gill. Two halves of pectoral arch well separated above.