CHAPTER IX 



CHARACTERS OF THE PHYLUM VERTEBRATA THE 

 LANCELET 



FROM your study of the frog you have already learnt 

 a good deal 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 following 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, con- 

 sisting of brain and spinal cord ; paired olfactory organs, 

 eyes, and auditory organs, which take on a close connec- 

 tion 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 nephridia ; a chambered heart and 



