IV. VERTEBRATA: PISCES 



505 



filament. The skin is covered with placoid scales, usually close together, 

 these being so small in some cases that the skin {shagreen) is used instead 

 of sandpaper. More rarely the scales are larger, and the spines, which 

 project from the skin, justify in size and form the term dermal teeth. 

 Such strong spines occur especially at the front of the dorsal fins {ichthy- 

 odolurites of paleontologists). The skeleton is cartilaginous, frequently 

 calcified on the outside. The calcification can also extend into the 

 vertebra;, producing star-like figures (Asterospondyli) or concentric circles 



Fig. 555. — Acanlhias vulgaris* dogfish ("from Claus). B, ventral fin; Br, pectoral 

 fin; Ks, gill clefts; n, nostril; R^, R-^ dorsal fins;6', heterocercal caudal fin; 5'/'/, spiracle. 



(Cyclospondyli). Since bone is lacking, the sharks have no upper jaws, 

 but bite with the pterygoquadrate. The amphicoelous vertebne (lack- 

 ing in Holocephali and several extinct groups,) 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. 555,5/1/). Except in the Holocephali the gill clefts 

 open separately, the hyoid arch being without an operculum. 



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

 ing elasmobranchs from Teleostomes. (i) The heart has a large conus, 

 with several rows of valves (fig. 553, ^4), but lacks a bulbus. (2) The 

 alimentary tract (fig 550, A) has a spiral valve, but lacks swim bladder 

 and pyloric cx'ca. (3) The sexual products are carried to the exterior 

 by the urogenital ducts except in Lccmargus which has abdominal pores. 

 The eggs escape from the follicles of the ovary (occasionally unpaired) 

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

 ostium tubas and the paired Mi'illerian ducts to the exterior. The 

 spermatozoa traverse the anterior part of the Wolffian body ('kidney'). 

 Sexual and urinary ducts open dorsally into the cloaca. 



Male elasmobranchs are distinguished by the presence of a copulatory 

 structure (mixiptcrygiiim) developed by enlargement of some radii of the ventral 

 fin (fig. 556, c). The large eggs, rich in yolk, are fertilized in the oviducts and 

 usually develop in uterine enlargements of the ducts. The embryos (fig. 540), 

 with long gill filaments protruding from the gill slits, are nourished by the yolk 



