60 COMPARATIVE ZOOLOGY. 
have nothing to do with ingestion or locomotion. Both 
Scorpions and Spiders have a soft upper lip, and a groove 
within the mouth, which serves as a canal while sucking 
their prey. The tongue is external, and situated between 
a pair of diminutive maxille. 
The mouth of Vertebrates is a cavity with a fixed roof 
(the hard palate) and a movable floor (the tongue and 
lower jaw), having a transverse opening in front, and a 
narrow outlet behind, leading to the gullet. Save in Birds 
and some others, the cavity is closed in front with lips, and 
the margins of the jaws are set with teeth. 
In Fishes, as in nearly all aquatic animals, the mouth is 
the common entry to both the digestive and respiratory 
organs; it is, therefore, large, and complicated by a mech- 
anism for regulating the transit of the food to the stomach 
and the aérated water to the gills. The slits leading to 
the gills are provided with rows of processes which, like 
a sieve, prevent the entrance of food, and with valves to 
keep the water, after it has entered the gills, from return- 
ing to the mouth. So that the mouths of Fishes may be 
said to be armed at both ends with teeth-bearing jaws. A 
few Fishes, as the Sturgeon, are toothless; but, as a class, 
they have an extraordinary dental apparatus—not only the 
upper: and lower jaws, but even the palate, tongue, and 
throat, being sometimes studded with teeth. Every part 
of the mouth is evidently designed for prehension. Lips 
are usually present; but the tongue is often absent, or very 
small, and as often aids respiration as ingestion. 
Reptiles have a wide mouth, even the_insect-feeding 
Toads and the Serpents can stretch theirs enormously. 
True fleshy lips are wanting; hence the savage aspect of 
the grinning Crocodile. With some exceptions, as Toads 
and Turtles, the jaws are armed with teeth. Turtles are 
provided with horny beaks. The tongue is rarely absent, 
but is generally too thick and short to be of much use. 
