THE ANCESTRY OF THE VERTEBRATES 77 



those who are familiar with common marine animals, 

 live for the most part a sedentary life fixed to objects 

 such as rocks or the piles of a pier, and they gain 

 their subsistence by passing a current of water, 

 charged with living organisms, through their pharynx 

 which is pierced by innumerable gill-slits. Like so 

 many fixed animals, their nervous system is degene- 

 rate, consisting of a single ganglion placed dorsally 

 to the mouth. This is the condition in the adult 

 state of the majority of Ascidians, but in the young 

 stages, after hatching from the egg, they lead an 

 active free-swimming existence, when they appear 

 with all the characters of a small tadpole, pre- 

 eminent among these characters being the possession 

 of a supporting rod, the notochord, which forms the 

 principal skeletal or supporting tissue, especially for 

 the tail which acts as the chief locomotive organ. 

 This notochord, which occurs in the young stages of 

 all Chordate animals, being the forerunner of the 

 vertebral column or backbone of the Vertebrates, 

 enables us to place the Ascidians in the Chordate 

 phylum with absolute certainty. When the Ascidian 

 tadpole becomes fixed and sessile, preparatory to 

 assuming the adult state, the notochord is absorbed 

 and disappears. 



The next class of Chordate animals is occupied 

 by a single form, Amphioxus or the Lancelet, and 

 this organism is of the most fundamental importance, 



