8 Vertebrata. 



the foremost of this set, the others are rudimental in 

 the higher animals. The visceral slit between the first 

 and second of these arches is the mouth ; the other 

 visceral slits remain either as the gill fissures in fishes, 

 or else become closed at an extremely early period 

 of embryonic life. The remnant of the first pair of 

 visceral slits behind the mouth we find in the form of 

 the ear passages in higher vertebrates. These visceral 

 arches never extend backwards behind the heart. 



5. Limbs and ribs. Vertebrate animals have 

 never more than four limbs, which are placed two in 

 front and two behind. The fore limbs are usually 

 placed a short way behind the head ; the hind limbs 

 at or immediately behind the posterior end of the 

 visceral cavity. Each limb has a bony or gristly axis 

 or skeleton, and this consists of two parts first, a 

 girdle or half-zone of bone, which is embedded in the 

 lateral muscles, and is often attached to the vertebral 

 column ; secondly, a limb ray or projecting part made 

 up of several sets of cartilages in a series. Some 

 vertebrates, like whales and some lizards, have only 

 two fore limbs and no hind limbs ; others, like boas 

 and pythons, have rudimentary hind limbs and no fore 

 limbs; others, like most of the snakes, have no limbs at 

 all. These limbs are always turned towards the 

 haemal or ventral side of the body. 



In the wall of the visceral cavity, following the 

 visceral arches, but quite separate from them, there 

 are usually long slender bones, jointed at the back 

 to the vertebral column, and forming supports for 

 the wall of this space. These bones are named 

 ribs, and the part of the body surrounded by them 



