118 VERTEBRATA. 



of the skull (tig. 571, //). The upper piece of the first arch, like- 

 wise, is sometimes more or less firmly applied along its whole length 

 to the skull, and when ossification takes place it becomes divided on 

 either side into an outer and inner series of pieces, the first including 

 the jugal, the maxilla, and praemaxilla, the latter the pterygoid and 

 palatine (fig. 575). These series of bones form the upper jaw and 

 the roof of the mouth. 



The lower jaw, which is primitively a simple cartilaginous arch 

 (Meckel's cartilage), also becomes replaced on either side by a 

 number of bones (articulare, angulare, clentary, etc.), of which 

 the clentary usually bears teeth and is the largest. 



The visceral arches which follow the mandibular arch and are also 

 connected with the skull are developed in the wall of the pharynx, 

 to which they bear the same relation that the ribs do to the thorax 

 and body cavity. The anterior arch (hyoid arch), the upper portion 

 of which in the lower Vertebrates serves as the suspensorium of the 

 jaw (hyomandibular), forms a support for the tongue, and the arch 

 of each side meets a median basal piece (os linyuale). The latter is 

 followed by a series of median unpaired bones (copulce), which connect 

 the following arches (branchial arches). The branchial arches are most 

 developed in the aquatic Vertebrates, in which they are separated by 

 the pharyngeal slits, and serve to bear the gills. In the air- 

 breathing Vertebrates they become more and more reduced, and 

 finally are only discernible in imperfect number as embryonic struc- 

 tures. The remains of the whole apparatus form the body and 

 cornua of the hyoid bone. 



Integument. The external skin of the Vertebrates is divided 

 into two very distinct layers, the epidermis externally and the cutis 

 internally. The latter is principally composed of a fibrous connective 

 tissue, with which muscular elements come into relation, without 

 however forming a complete dermal-muscular envelope as in the 

 Annelids.- 



When the dermal muscles have a considerable extension over large 

 surfaces, they serve exclusively to move the skin and its manifold 

 appendages, but are not used for the movements of the trunk, which 

 are produced by a highly -developed muscular system surrounding the 

 skeleton. The cutis is continued into a deeper, more or less loose 

 layer, the, subcutaneous connective tissue, but its more superficial 

 part is tolerably compact, and contains not only various pigments, but 

 also blood-vessels and nerves. At its upper surface the cutis is raised 

 into small conical papilla^, which are covered by the epidermis and 



