CHORDATA 139 



invertebrates. All have a portal vein, carrying blood 

 through the liver; all have lacteals and lymphatics. 

 The blood is red, and contains both kinds of corpuscles. 

 The teeth are developed from the dermis, never from 

 the cuticle, as in mollusks and arthropods; the jaws 

 move vertically, and are never modified limbs. Except 

 in the lowest forms the liver and kidneys are always 

 present. The respiratory organs are either gills or 

 lungs, or both. Vertebrates are the only animals which 

 breathe through the mouth cavity. 



The nervous system has two marked divisions : the 

 cerebrospinal, presiding over the functions of animal 

 life (sensation and locomotion); and the sympathetic, 

 which partially controls the organic functions (digestion, 

 respiration, and circulation). . In no case does the gullet 

 pass through the nervous system, as in invertebrates, 

 and the mouth opens on the side opposite to the brain. 

 Except in the lowest members of this group probably 

 none of the five senses is ever altogether absent. The 

 form of the brain is modified by the relative develop- 

 ment of the various lobes. In the lower vertebrates, 

 the cerebral hemispheres are small in certain fishes 

 they are actually smaller than the optic lobes in the 

 higher, they nearly or quite overlap both olfactories 

 and cerebellum. The brain may be smooth, as in most 

 of the cold-blooded animals, or richly convoluted, as in 

 man. 



There is no skull in Amphioxus. In the Cyclosto- 

 mata and Elasmobranchii it is cartilaginous. In other 

 fishes it is cartilage overlaid with bone. In amphibians 

 and reptiles, it is mingled bone and cartilage. In birds 

 and mammals, it is mainly or wholly bony. The human 

 skull contains fewer bones than the skull of most 

 animals, excepting birds. The skull of all vertebrates 

 is divisible into two regions : the cranium, or brain case, 



