THE TRUE FISHES. 



159 



easily bent or cut with a knife. The bones that protect 

 the gill-openings in bony fishes are wanting, the gills being 

 mere slits,* from five to seven pairs, with intervening 

 straps. The lobes of the tail are generally unequal 



FIG. 196. A, egg of a shark, showing the anchor filaments, 

 shark, showing the external gills (br). 



B, embryo 



Order I. Plagiostomi. Nurse-Sharks (Scymruda). 

 The nurse or sleeper is a sluggish shark found upon the 

 eastern coast of North America, occasionally attaining a 

 length of twenty feet. A nurse-shark of southern Florida 

 is extremely dark in color. The latter frequent the sandy 

 shoals in droves, and can always be seen asleep or quiet 

 upon the bottom. 



NOTE. Dr. Bennett discovered in Australian seas a small shark 

 (Squalus fulgens\ probably allied to this family, that was luminous 



* In embryo sharks (Fig. 196, B) the gills are external, as in the 

 tadpole, etc. 



