2 THE ANATOMY OP VEKTEBEATED ANIMALS. 



Again, in the partition between the cerebro-spinal and 

 visceral tubes, certain structures, which are not represented 

 in invertebrated animals, are contained. During the em- 

 bryonic condition of all vertebrates, the centre of the parti- 

 tion is occupied by an elongated, cellular, cylindroidal mass 

 — the notochord, or chorda dorsalis. And this structure per- 

 sists throughout life in some Vertebrata ; but, in most, it is 

 more or less completely replaced by a jointed, partly fibrous 

 and cartilaginous, and partly bony, vertebral column. 



In all Vertebrata, 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 develop- 

 ment, a series of thickenings, parallel with one another 

 and transverse 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 pharyngeal cavity, temporarily or permanently, 

 in communication with the exterior. Nothing correspond- 

 ing with these arches and clefts is known in the Inverte- 

 brata. 



A vertebrated animal may be devoid of articulated limbs, 

 and it never possesses more than two pairs. These are 

 always provided with an internal skeleton, to which the 

 muscles moving the limbs are attached. The limbs of 

 invertebrated animals are commonly more numerous, and 

 their skeleton is always external. 



When invertebrated animals are provided with masti- 

 catory organs, the latter are either hard productions of the 

 alimentary mucous membrane, or are modified limbs. Ver- 

 tebrated animals also commonly possess hard productions 

 of the alimentary 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 



