REPORT 0:^" THE TUNICATA. 
387 
appendage. In Morchellioides affinis (see p. 177) the post-abdomen terminates in 
distinct vessels which run for a short distance through the test and terminate in bulbs 
(see PI. XXI Y. fig. 17, v.ap.). 
In the Botryllid£B the system of branched and anastomosing vessels in the test has 
become greatly enlarged, and probably forms an accessory organ of respiration.^ The 
explanation of this system being so much more developed in the Botryllidm than in 
the other Compound Ascidians is to be found in the evolution of the group. The 
Botryllidse have probably sprung from the Simple Ascidians after these had acquired 
well-marked systems of respiratory vessels in their tests, while most of the other 
Compound Ascidians originated at an earlier point, while the vessels were stiU in the 
form of gemmiparous stolons (see under Phylogeny, below). 
In those Distomidse where the colony has become pedunculated {e.g., Colella) the 
young Ascidiozooids are added at the base of the head, and those at the summit are the 
oldest ; but in one species of the Polystyelidse, Goodsiria placenta, the older Ascidio- 
zooids seem to be next the peduncle, while the younger ones are at the upper end of the 
colony. In Ccelocormus huxleyi also the older Ascidiozooids seem to be in what must be 
regarded as the basal part of the colony. In all these cases the relative positions of the 
Ascidiozooids, and, to a certain extent, the shape of the colony, depend upon the method 
and the region in which gemmation is carried on. 
Phylogeny. 
The detailed investigation of the structure of the various groups of Simple and 
Compound Ascidians, and the consideration of their relationships, have gradually led 
me to the conclusion that the Ascidise Compositse are polyphyletic in origin, being 
composed of several branches which have arisen from the Simple Ascidians at difierent 
times. Consequently, in order to explain the probable phylogeny of the Com|)ound 
Ascidians, it will be necessary to refer to that of the Simple Ascidians discussed at the 
end of the first part of this Eeport.^ 
The ancestral Simple Ascidian there described, and which I regard as being also the 
common ancestor of all the Compound Ascidians, was probably derived from the Proto- 
Tunicata^ after the separation of the Proto-Thaliacea and the primitive Appendiculariidse 
(see fig. 11, p. 388). It was the first fixed Ascidian — the ancestral Tunicates, and the 
primitive Appendiculariidse, Salpidse, and Doliolidse derived from them being all free 
^ See Herdman, On a New Organ of Respiration in the Tunicata, Proc. Lit. and Phil. Soc. Liverpool, vol. xxxix. 
p. 39, 1884-85. 
^ In vol. vi., 1882, part xviii. p. 285. 
^ Tor my views as to the origin of the Proto-Tunicata and their branches see A Phylogenetic Classification 
of Animals, Macmillan, 1885. A more detailed account of the probable phylogeny of the Tunicata, and of the 
relations of the Thaliacea to the other groups, will be given in the third and concluding part of this Report. 
