

CHAPTER IX 



CHARACTERS OF THE PHYLUM VERTEBRATA THE LANCELET 



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 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 follow- 

 ing essential characters. 



They all possess : a vertebral column 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 jaws ; a dorsal, hollow, nervous system, consisting of 

 brain and spinal cord ; paired olfactgry 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 gill-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 heart and red blood-corpuscles ; a liver, 



