PRIMITIVE VERTEBRATES 



299 



form and the typical Vertebrate characters. It is possible that 

 these are primitive forms which retain the features distinctive 

 of the ancestral Ascidians, but it is also possible that we have 

 a case of animals which have dropped the adult stage out of 

 their life-history, just as the Mexican Axolotls appear to be 

 doing (see p. 249). 



Ascidians may be divided into Fixed and Free-swimming 

 forms, each of which groups can be again split up into Simple 



EAR 



BRAIN 



NOTOCHORD 



INTESTINE. 



GILL CLEFT 



HEART- 



NERVE CORD- 



STOMACH' 



Fig. 173. Diagrammatic drawing of Appendicularia (much enlarged), as seen from left side, with 

 dorsal surface to right. D T, Dorsal tubercle. 



and Colonial species. The last expression needs explanation, as 

 it involves a phenomenon of which no instance is furnished by 

 the animals previously considered. All these are propagated 

 solely by means of eggs, but in Ascidians and many of the 

 lower Invertebrates there may be increase by means of out- 

 growth of buds (gemmation) or by the bodily splitting (fission) 

 of individuals to form others. As this suggests similar processes 

 among plants, it is commonly known as vegetative propagation. 

 A collection of animals which have been formed in this way, and 

 which remain united together, constitute a colony, or are said to 

 be colonial. In Ascidians colonies may be formed by means 

 of budding. 



