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 casca 

 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- 

 curs, and this may bear 

 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 

 ampullse 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 



Fig. 235. Relations of gill clefts in the elas- 

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

 spiracle. 



