of Edinburgh, Session 1885-8G. 445 
pound Ascidians. The DipJosomidae were derived from the ancestral 
Didemnidae. 
The remaining two families of the Compound Ascidians, the 
Botryllidae and the Polystyelidae, were derived independently from 
the ancestral Simple Ascidians, and are therefore not closely allied 
to the Polyclinidae and the Distomidae. Probably the Botryl- 
lidae arose from the primitive Cynthiidae which were derived from 
the Ascidiidae. The Polystyelidae, on the other hand, form a 
modified offshoot from the Styelinae. The new “Challenger” genus 
Chorizocormus shows how the transition may have been effected from 
such a Simple Ascidian as Polycarpa to such a Compound Ascidian 
as Goodsiria. 
The Proto- Ascidiacea were probably colonial forms, and gemma- 
tion was retained by the Clavelinidae and by the typical Com- 
pound Ascidians (Distomidae, &c.) derived from them. The power 
of forming colonies by budding was lost, however, by the primi- 
tive Ascidiidae (Simple Ascidians), and must, therefore, have been 
regained independently by the ancestral forms of the Botryllidae and 
the Polystyelidae derived from the Simple Ascidians. In the Poly- 
styelidae the differentiation of the colony has not gone so far as in 
most other groups of the Compound Ascidians, and the Ascidiozooids 
have not become arranged in true systems. 
The ancestral Cynthiidae split into two lines of descent — one 
leading to the Styelinae and the Polystyelidae, and the other to the 
primitive Cynthinae. This latter group is represented most nearly at 
the present day by the genus Cynthia , while the Bolteninae form a 
side branch. The Molgulidae were also derived from the primitive 
Cynthinae ; they have undergone considerable modification. 
In the paper the details of the changes which the various families 
and genera have probably undergone during their evolution are dis- 
cussed, and the more important hypothetical ancestral forms are 
considered. The two most striking conclusions which have been 
arrived at are — (1) the relationship of Pyrosoma with the Disto- 
midae through Coelocormus ; and (2) that the Compound Ascidians 
are a polyphyletic group derived from the ancestral Simple Ascidians 
at three distinct points. 
