ELASMOBRANCHS. 



233 



distance behind the tip of the snout. The body is covered with 

 placoid scales, which are usually small, and form shagreen, for- 

 merly much used by cabinet-makers in place of sandpaper. The 

 pelvic fins are always abdominal in position. 



The jaws are armed with acutely pointed teeth or with flat- 

 tened crushing-plates. The oesophagus is ciliated ; the stomach 

 shaped like the letter 

 J, and no pyloric caeca 

 occur. The intestine 

 is provided with a well- 

 developed spiral valve, 

 and the rectum bears 

 a finger-shaped rectal 

 gland on its dorsal sur- 

 face. A cloaca is pres- 

 ent. 



The gill clefts are 

 usually five in number, 

 six or seven in some 

 lower forms. They 

 open freely to the out- 

 side, no operculum be- 

 ing developed. The 

 gills are attached their 

 whole length to the 

 interbranchial septum. 



Usually a spiracle OC- ^ IG< 2 35' Relations of gill clefts in the efas"- 



, , , . mobranchs. h. hyoid arch ; m, mandible : s. 



curs, and this may bear spiraclee 

 a pseudobranch (p. 23). 



The hemispheres of the brain are united, and the olfactory- 

 lobes are separated from the cerebrum by an elongate olfactory- 

 tract. The twixt brain is short, and an optic chiasma occurs. 

 The lateral line system is well developed, and in the skates be- 

 comes greatly branched. On the head are numerous sensory 

 ampullae filled with jelly. 



The skeleton is cartilaginous ; but in many cases it is ren- 

 dered more dense by the deposition of lime, which, however, 

 never takes the shape of bone corpuscles, there being a sharp 



