112 VERTEBRATA. 



form the brain and with the first portion of the alimentary canal. 

 The canal formed by the neural arches is here dilated to form the 

 cranial capsule, on the ventral side of which are placed cartilaginous 

 arches known as the visceral arches, of which the anterior pair 

 constitutes the mandibular apparatus, is armed with teeth and 

 surrounds the entrance to the alimentary canal (fig. 571). 

 The mandibular arch is followed by a number of arches which 

 surround the pharynx ; the first of these is the hyoid arch and the 

 rest are the branchial arches. 



The part of the body behind the head may be divided into two 

 regions: (1) an anterior region the trunk proper in which the 

 peritoneal or body cavity lined by the peritoneal men-brane is placed ; 

 the vertebrae in this region bear ribs ; (2) a posterior region or tail, 



S 



FIG. 571. Head and anterior region of the vertebral column of Acanthias (after Owen). 

 K, Body of vertebra; 0, neural arch; S, intercalated piece; Pq, Palatoquadrate ; 

 Lk, labial cartilage ; Zb, hyoid arch ; Kb, branchial arch ; Sg, shoulder-girdle. 



in which there is no body cavity, and the hsemapophyses unite with 

 each other to enclose a canal (containing the caudal vessels). This, 

 the most simple form of segmentation of the trunk, is confined to the 

 lower Vertebrates which propel themselves by the flexion and undu- 

 latory movements of the vertebral column, and, like the Annelids, live 

 in water, in mud, or in the earth, or even creep after the manner of 

 snakes on the surface of the earth. 



In the higher Vertebrates, however, in which, as in the Arthropods, 

 the function of locomotion is discharged by paired appendages, the 

 movements of the chief axis are reduced and in many regions are 

 altogether absent. In the Vertebrata there are only two pairs of 

 limbs, an anterior pair and a posterior pair. In the lower Verte- 

 brata they have the form of fins and play but a subordinate part in 

 locomotion. In such cases, therefore, the vertebral column retains 



