288 TADPOLES OF THE SEA 



placed beyond doubt. Each piece of evidence multiplies 

 enormously the chances against the resemblance being due 

 to coincidence, and not to identity of race. So it is with 

 our tadpoles of the sea. They do not merely resemble 

 vertebrates they are vertebrates ; they belong to that 

 great line of the animal pedigree. The strange immobile 

 mollusc-like sacs known as " Ascidians " or " sea-squirts," 

 so unlike the little tadpoles from which they gro\v, are 



MUM 



NOTOCHORD ^ 





FIG. 37. Two stages in the fixation of the Ascidian tadpole by its 

 head to a rock and its subsequent degeneration or simplification. 

 In the upper figure the tail is seen to be withering and the brain 

 no longer is elongated to form a spinal cord. In the lower figure 

 the shape of the young animal is much changed, the tail has 

 almost entirely withered up, and the region affixed to the rock has 

 grown relatively large. 



simplified or " degenerate " as compared with that so-called 

 " larval " or transient youthful stage of growth. Though 

 we only find this history in the growth from the egg of a 

 few of the Ascidians, yet all those which never pass 

 through a tadpole stage of growth, have to be embraced 

 in our conclusion. The Ascidians are simplified sac-like 

 vertebrates. 



