I 10 TUNICATA CHAP. 



indicated by a line diverging from near the point (3) in the 

 phylogenetic diagram below. 



General Conclusions. 



The following diagram is a graphic representation of the 

 genetic affinities, or what is now generally supposed to have been 

 the probable course of phylogeny of the Tunicata. It will be 

 noticed that it shows (1) the Proto-Tunicates arising from Proto- 

 Chordata, not far from the ancestors of Amphioxus (see also, this 

 vol. p. 112); (2) that the Larvacea are regarded as the most 



Molgulidae 



primitive section of the group ; (3) that the Thaliacea (Dolio- 

 lidae and Salpidae) are supposed to be derived not directly from 

 primitive pelagic forms, but through the early fixed Ascidians, 

 not far from (4) the ancestral compound Ascidians, which gave 

 rise to the Pyrosomatidae; (5) that the Ascidiidae and other 

 higher Simple Ascidians are derived, like the Compound Ascidians, 

 from ancestral Clavelinidae; and ('6), that the Ascidiae Compositae 

 are polyphyletic, the Holosomata (Botryllidae and Polystyelidae) 

 being derived from ancestral Simple Ascidians independently 

 of the Merosomatous families. 



The Tunicata are remarkable for the variety in appearance, 

 structure, and life-history which they present. No group illus- 

 trates in a more instructive " manner so large a number of 

 important biological principles and phenomena. They show 

 solitary and colonial forms, fixed and free, pelagic and abyssal. 

 The development is in some cases larval and with metamorphosis, 

 in others abbreviated and direct. Persistent traces of ancestral 

 characters are seen in the embryonic and larval stages, while the 

 adults present the most varied secondary adaptations to littoral, 



