570 CHORD ATA. 



dorsal fins (ichthyodolurites of paleontologists). The skeleton is 

 cartilaginous, frequently calcified on the outside. The calcifica- 

 tion can also extend into the vertebrae, producing star-like figures. 

 Since bone is lacking, the sharks have no upper jaws, but bite with 

 the pterygoquadrate. The amphicoelous vertebras (lacking in the 

 Holocephali and the extinct Cladoselachii, Ichthyotomi, and 

 Acanthodidae), have neural arches, small ribs, and intercalaria. 

 The number of gill arches and clefts varies between five and seven, 

 the first cleft lying between the hyoid and the first branchial arch. 

 Besides, most elasmobranchs have a spiracle and pseudobranch 

 (fig. 598, Spl}. Except in the Holocephali the gill clefts open sep- 

 arately, the hyoid arch being without an operculum. 



In the visceral anatomy these points are of importance as dis- 

 tinguishing elasmobranchs from Teleostomes. (1) The heart has a 

 large conus, with several rows of valves (fig. 596, A), but lacks a 

 bulbus. (2) The alimentary tract (fig. 593, A) has a spiral valve, 

 but lacks swim bladder and pyloric caeca. (3) The sexual products 

 are carried to the exterior by the urogenital ducts. The eggs 

 escape from the follicles of the ovary (occasionally unpaired) by 

 dehiscence into the body cavity, and from thence by the unpaired 

 ostium tubae and the paired Miillerian ducts to the exterior. The 

 spermatozoa traverse the anterior part of the Wolffian body (' kid- 

 ney'). Sexual and reproductive ducts open dorsally into the 

 cloaca. 



Male elasmobranchs are distinguished by the presence of a copu- 

 latory structure (mixipterygium) developed by enlargement of some radii 

 of the ventral fin (fig. 599, c). The large eggs, rich in yolk, are fertilized 

 in the oviducts and usually develop in uterine enlargements of the ducts. 

 The embryos (fig. 582), with long gill filaments protruding from the gill 

 slits, are nourished by the yolk in the yolk sac. In Mustelus and Car- 

 charias, as Aristotle knew, there is the formation of a placenta, which 

 differs from that of the mammals in that the embryonic blood supply 

 arises from the blood-vessels of the yolk sac and are not allantoic. There 

 are oviparous elasmobranchs, and in these the egg is surrounded by 

 albumen and a shell, but these eggs differ from those of birds in that the 

 skull is horny and is usually drawn out at the four corners, sometimes 

 with threads for attaching the egg to plants, etc. 



Order I. Sjelachii. 



With the notochord more or less completely replaced by verte- 

 bral centra; no dermal bones. 



Sub Order I. DIPLOSPONDYLI. Gill slits lateral, six or seven in 

 number, a single dorsal fin. Chlamydoselaclms with terminal mouth. 

 Hexanchus,* mouth normal, six gill slits ; Heptanchus, seven gill slits. 



