8 THE ANATOMY OF YERTEBRATED ANIMALS. 



ceral tubes, certain structures, whicli are not represented in 

 invertebrated animals, are contained. During' the embryonic 

 condition of all vertebrates, the centre of the partition is occu- 

 pied by an elongated, cellular, cylindroidal mass — the noto- 

 chord, or chorda dorsalis. And this structure persists through- 

 out life in some Vertehrata ; but, in most, it is more or less 

 completely replaced by a jointed, partly fibrous and cartilag- 

 inous, and partly bony, vertebral column. 



In all Vertehrata^ that part of the wall of the visceral tube 

 which lies at the sides of, and immediately behind, the mouth, 

 exhibits, at a certain stage of embryonic development, a 

 series of thickenings, parallel with one another and trans- 

 verse to the axis of the body, which may be five or more in 

 number, and are termed the visceral arches. The intervals 

 between these arches become clefts, which place the pharyn- 

 geal cavity, temjDorarily or permanently, in communication 

 with the exterior. Nothing corresponding with these arches 

 and clefts is kno^^m in the Invertehrata. 



A vertebrated animal may be devoid of articulated limbs, 

 and it never possesses more than two pairs. These are always 

 pro^^ded with an internal skeleton, to which the muscles mov- 

 ing the limbs are attached. The limbs of invertebrated ani- 

 mals are commonly more numerous, and their skeleton is 

 always external. 



When invertebrated animals are provided with masticatory 

 organs, the latter are either hard productions of the alimentary 

 mucous membrane, or are modified limbs. Vertebrated ani- 

 mals also commonly possess hard productions of the alimen- 

 tary mucous membrane in the form of teeth ; but their jaws 

 are always parts of the walls of the parietes of the head, and 

 have nothing to do with limbs. 



All vertebrated animals have a complete vascular system. 

 In the thorax and abdomen, in place of a single peri-visceral 

 cavity in communication with the vascular system, and serving 

 as a blood-sinus, there are one or more serous sacs. These 

 invest the principal viscera, and may or may not communicate 

 witli the exterior — recalling, in the latter case, the atrial cavi- 

 ties of Mollusca. 



In all Vertehrata^ except Amjyhioxus^ there is a single 

 valvular heart, and all possess an hejyatic portal system y the 

 blood of the alimentary canal never being wdiolly returned di- 

 rectly to the heart by the ordinary veins, but being more or 

 less completely collected into a trunk — the portal vein, which 

 ramifies through and supplies the liver. 



