CriAPTER VIII 



CHARACTERS OF THE PHYLUM VERTEBRATA — AMPHIOXUS.- 



From your study of the frog, you have learnt something 

 about a vertebrate animal, and we will now examine 

 a few more examples of the phylum Vertebrata, which, 

 as we have seen (p. 219), includes several classes, the 

 chief of which are — the Pisces, Amphibia, Reptilia, Aves, 

 and Mammalia. Leaving aside the lampreys and their 

 allies, which present certain peculiarities and are therefore 

 placed in a class by themselves, these all agree with" one 

 another and resemble the frog in the following essential 

 characters. 



They all possess : — a vertebral column— ox at any rate a 

 notochord (p. 203), which is nearly always replaced by a 

 vertebral column in the adult — and a skull with upper and 

 lower jaws ; a hollow, dorsal, nervous system, consisting of 

 brain and spinal cord; paired olfactory organs, eyes, and 

 auditory organs, which take on a close connection with the 

 skull ; a pharynx, which, at an early stage at any rate, is 

 perforated by a small number (never more than seven) of 

 paired giU clefts (p. 204) ; a mouth which is ventral and 

 anterior, and an anus which is ventral and posterior ; kidneys 

 which are composed of numerous urinary tubules or neph- 

 ridia ; a chambered Mart and red blood-corpuscles ; a liver, 



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