59 



is limited to the posterior part of the body, and does not 

 extend forwards into the region occupied by the chief parts 

 of the nervous system and alimentary canal. Whether this 

 is the result of degeneration or indicates the primitive con- 

 dition of the notochord in the Proto-Chordata is uncertain. 

 Probably it is to some extent at least the result of degenera- 

 tion or adaptation of the primitive Tunicata to surrounding 

 conditions. 



It may readily be imagined that these free-swimming 

 surface organisms might find it advantageous that the 

 alimentary canal, which performed both nutritive and respi- 

 ratory functions, and the main part of the nervous system 

 in connection with which sense organs were being devel- 

 oped, should be as much as possible concentrated in the 

 anterior part of the body so as to leave the posterior part free 

 to become modified into an efficient locomotory organ. 

 Under such circumstances it would be natural that the 

 notochord, the sole internal skeleton, should be restricted to 

 the posterior part of the body where it would form an axis 

 around which the muscles were placed. 



From this ancestral Tunicate (fig. 20), represented at the 

 d 



Fig. 20. Hypothetical ancestor of the Tunicata. yn, mouth; c, notochord; 

 b, anterior respiratory part of the aUmentary canal; d, dorsally placed nervous 

 system; a, anus; s, one of the segments or metameres; i, intestine; g, anterior 

 ganglionic part of the nervous system ; p, prse-oral lobe forming front of head ; 

 n^, anterior nephridia forming neural gland (hypophysis cerebri?). 



present day by the Appendiculariidae two diverging lines 

 seem to have started, both showing degeneration. In the 

 first the organism remained free-swimming but acquired the 

 power of passing currents of water through its respiratory 

 system of cavities in such a way as to propel itself through 



