61 



of primitive Social Ascidians gradually lost the power of 

 reproducing by gemmation, and so became the first solitary 

 or Simple Ascidians.* 



The long line extending from the point of separation 

 of the most primitive Chordate from the Enteropneusta 

 upwards to the place where the ancestral Tunicata diverged, 

 indicates the considerable amount of evolution which must 

 have taken place in the series of Proto-Chordata. It was 

 during this period that the notochord and other character- 

 istic structures of the Chordata, some of which are referred 

 to above, were gradually acquired. 



The Cephalochorda (Amphioxus) are, like the Tunicata, 

 degenerate, and probably their ancestors diverged from the 

 main stem nearly at the same point. As the notochord 

 extends to the anterior end of the body in the Cephalo- 

 chorda, it is highly probable that its restriction to the 

 posterior part in the Tunicate larva and the Appendicu- 

 lariidae is, as suggested above, the result of adaptation. The 

 anterior part of the nervous system is in a degenerate 

 condition in Ainphioxus, and sense organs are very feebly 

 developed. The whole head is in a most rudimentary 

 condition, probably as the result of degeneration. 



Above the Proto- Chordata! from which the Cephalo- 

 chorda arose, important changes took place resulting in the 

 formation of a series of ancestral forms which Balfour has 

 called the Proto-Vertebrata. In this group the notochord 

 was no longer the only skeleton, neural arches extending 

 upwards at the sides of the dorsal nervous system had made 

 their appearance in the mesoderm. The anterior part of the 

 nervous system became more highly developed so as to form 



* For the phylogeny of the Simple Ascidians, see Herdman, Challenger 

 Zoological Reports, vol. vi, part xvi, p. 286 (1882). 



t In this region of the primitive Chordata and the lower Vertebrata the 

 phylogeny given by Balfour has been followed (see Go7np. Embryol., vol. ii, 

 p. 271). 



