26o CHARACTERS OF VERTEBRATE ANIMALS 



" visceral skeleton ". Regarding the last two a little more may- 

 be said, as they have much to do with the visceral arches and 

 clefts which are so characteristic of Vertebrates, and have been 

 so often alluded to in the foregoing pages (see pp. 62 and 242). 



Examination of a young embryo of the Dog-Fish will show 

 that on each side of the head there are seven oblique bar-like 

 thickenings and six slits occupying the interspaces between them. 

 These are respectively termed visceral arches and clefts, the 

 hindermost five having the special name of gill arches and clefts 

 because the gills are developed in connection with them. The 

 first arch is the mandibular arch and the second the hyoid arch, 

 while the slit between them is naturally called the hyo-mandibular 

 cleft and is no other than the spiracular cleft of the adult. These 

 various arches are traversed and supported by firm jointed rods 

 forming part of the internal skeleton. The mandibular arch is 

 so called because it becomes the mandible or lower jaw, while 

 the upper jaw is a forward outgrowth from it. The skeleton of 

 the rest of the arches constitutes the visceral skeleton, which 

 stiffens the gill region and prevents the gill-slits from becoming 

 closed, giving also firm points of attachment to many muscles. 

 A special function is performed by the upper joint of the skeleton 

 of the second or hyoid arch, this being a stout piece of cartilage 

 (hyomandibular cartilage) which slings the jaws to the main 

 skull. In Vertebrates higher than Fishes this cartilage loses its 

 original function, for the jaws are directly attached to the skull, 

 and most probably some or all of the little ear-bones, which help 

 to conduct sound-waves across the drum of the ear in such 

 higher Vertebrates, correspond to this cartilage. This is one of 

 the best examples known of a change of function. The rest of 

 the visceral skeleton, here so important in connection with the 

 gill -clefts, dwindles in air-breathing Vertebrates to the "hyoid 

 apparatus", which has been so often mentioned (see pp. 29, 193, 

 and 239) as supporting the base of the tongue in these forms, 

 and to the cartilages which support the voice-box or larynx. In 

 the life-history of the frog the passage from a well-developed 

 visceral skeleton in the gill-bearing tadpole to such remnants in 

 the adult lung-possessing animal can be traced step by step. 



In the vertebral column it is only possible to distinguish 

 between trunk-vertebrae and tail-vertebrse, and all these possess 

 biconcave centra, which may be regarded as the most primitive 



