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. 
Fic. 196.—A, egg of a shark, showing the anchor filaments. B, embryo 
shark, showing the external gills (67). 
Order I. Plagiostomi. Nurse-Sharks (Scymnide). 
—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. 
NotEe.—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. 
