68 



LOWER VERTEBRATES. 



Class IV. — ELASMOBRANCHII. 



In different works on natural history, three separate names are used for that group 

 of iish-like vertebrates which embraces the sharks and skates. These are Selachii 

 ■ (derived from the Greek word for shark), Chondropterygii (alluding to the cartilagi- 

 nous nature of the skeleton of the fins), and Elasmobranchii (plate- or blade-like gills). 

 In some respects the groujD is highly organized, while in others its members stand on 

 a low grade of development. This has resulted in the assignment of the Elasmo- 

 branchs to A'ery different positions in systematic arrangement of the vertebrates, 

 according as one set of structures or another was made prominent. It is, however, 

 certain that the sharks and skates have diverged less from the main vertebrate stem 

 than ha\'e the Teleosts. The skeleton in the Elasniobranchs is cartilaginous, and the 

 separate elements of the skull are united without sutures ; pectoral and ventral fins 

 are present, the former large and well developed ; the caudal fin is usually heterocercal, 

 the vertebral column extending into the upper lobe ; the mouth is on the lower sur- 

 face ; the gill pouches are usually five in number, but they may be increased to six or 

 seven, the gill slits corresponding; the optic nerves unite in a chiasma; the intestine 

 has a spiral valve, and the conus arteriosus has several rows of valves. 



The external surface of the body may be naked, but more usually it is covered with 

 calcified papillas which in origin and structure are comparable to teeth. Sometimes 

 these ossified papillae are small and closely set, thus forming shagreen ; sometimes they 

 are larger and more widely separated, forming a hardened armor for the body or for 

 certain portions of it. These ' placoid ' scales are frequently ornamented with beauti- 

 ful patterns of sculpture. Sometimes these dermal jjlates are united to form spines, 

 which usually occur in front of the dorsal fins, but may occasionally be found in other 

 parts of the body. These dermal spines are frequently found in a fossil condition, and 

 are collectively known under the name ichthyodolurites. 



The internal skeleton varies much in the extent of its development, and in the 

 relative j^roportion of its parts. In Chinicera a slight calcification, arranged in 

 segments, appears in the sheath of the notochord ; and all degrees between this 



and the condition where 

 the bodies are more or less 

 ossified, and have concave 

 anterior and posterior sur- 

 faces, may be found. In 

 the rays, ossification in the 

 anterior end of the verte- 

 bral column has converted 

 the whole into a bony mass. 

 Neural and haemal arches 

 may occur. 



The skull is cartilaginous 

 and rudimentary, and usual- 

 ly has large membranous fontanelles on its upper surface. In Chimoera the palato- 

 qiiadrate and the suspensorium are united to each other and to the skull, while in the 

 Plagiostomi they are distinct and movably articulated with the skull, the manner of 

 their articulation having recently been made a feature of taxonomic importance. 



Fig. 55. — SkuU and visceral arches of adog-Hsb ; km, hyoniandibular ; m, 

 lower jaw (Meckel's cartilage with dotted outline) ; mp, nietapterygoid 

 ligament ; n, nasal capsule ; q, pterygoquadrate ; s, supraorbital ridge ; 

 sp, spiracle ; t, palato-trabecular ligament ; 1, 2, 3, 4, 6, labial cartilages. 



