38 THE ORIGIN OF MAN 



(tunicates). These animals are greatly degen- 

 erate in adult life, due to their sedentary habit, 

 and their vertebrate affinity is only shown in 

 the earliest stages of growth. They are on border 

 line between the invertebrates and the verte- 

 brates. Their tiny larvae are free and active 

 animals and what is so important in these larvae 

 of the sea-squirts is that they have in addition to 

 the hemichordate structures a true notochord in 

 the tail, the notochord being the rudiment out of 

 which the vertebrated backbone of the higher 

 vertebrates has evolved. Sea-squirts are there- 

 fore spoken of as primitive chordates, that is, ver- 

 tebrate animals with the most primitive type of 

 backbone, which, however, is not bone, nor even 

 cartilage, but a gelatinous unsegmented material. 

 The spinal cord or trunk nerve lies above or dor- 

 sally to the notochord. 



Lancelets represent the third step toward the 

 perfection of the vertebrates, but are an offshoot 

 from the main line of ancestors to the fishes. 



