74 ORIGIN OF CHORDATES in. 9- 



but the fact that they differ in many ways from known ascidian 

 tadpoles does not invalidate the hypothesis; it would be expected 

 that many special features would be developed during evolution 

 after the paedomorphosis. Garstang, however, believed that there 

 is sufficient evidence to show that the Larvacea are related to the 

 Doliolidae and suggested an ingenious hypothesis by which the appendi- 

 cularian home could be derived from the doliolid test, the animal 

 itself remaining attached at the front end by gelatinous threads, which 

 came to make the filter tubes (Fig. 44). 



Fig. 44. Sequence of stages by which the Larvacea may have 

 been evolved from a doliolid type. 



A, Thaliaccan type of individual in its test (t). b, Paedomorphosis has 

 occurred so that a tailed creature is found in the test; g. gill-slit, c, The 

 tadpole has moved away from the inhalant aperture, leaving a series 

 of threads that become the filter pipes (/./>.), the inhalant aperture 

 becoming exhalant and vice versa. (After Garstang.) 



The tail is a highly developed organ, serving for locomotion, nutri- 

 tion, and in the building of the house. It has a wide, continuous fin 

 and is supported by a notochord of 20 cells. Bands of 10 large striped 

 muscle-cells extend down each side, giving an appearance that has 

 been compared with metameric segmentation. The small number of 

 the cells makes any such comparison very difficult. Moreover, the 

 muscles are not developed from anything resembling myotomes. The 

 nerve-cord is a hollow tube with ganglionic thickenings, each con- 

 taining one to four nerve-cells. From these cells fibres proceed to 

 the muscles and to the skin in a series of roots that usually remain 

 separate, the motor being more dorsal. 



10. The formation of the chordates 



We can now recapitulate the points that we have established about 

 the origin of the chordates and attempt to piece together the evidence 

 to show the sequence of events that led to the production of a free- 



