CHAPTER Aill 



CHAKACTERS 01' THE I'HYIX'-M \JiRXt.l:RATA - AMPHIOXUS. 



From your study of the frog, )ou have learnt something 

 about a vertebrate animal, and we will now examine 

 a few more examples of the phylum ^^ertebrata, which, 

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

 chief of which are — the Pisces, Ampliibia, Reptilia, Avcs, 

 and Maininalia. 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 coluiint — or 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 jaivs \ a holloiv, 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 (ne\'er more than seven) of 

 paired gill-clefts (p. 204) ; a month' which is ventral and 

 anterior, and an anus which is ventral and ]_)Osterior ; kidneys 

 which are composed of numerous urinary tubules or neph- 

 ridia ; a chambered heart and red blood-corpuscles ; a livery 



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