EEPOET ON THE TUNICATA. 
37 
whole the Botryllidse are feebly represented in the collections compared with most of the 
other groups and compared with the part they play in the British Ascidian fauna. 
The four genera may be distinguished by the following characters : — 
BoTRYLLIDiE. 
Systems circular in 
outline. 
Systems elongated, or 
branched irregularly. 
Colony thin, 
incrusting. 
I 
Botrijllus. 
Colony thick 
and fleshy. 
Polycyclus. 
Colony thin, Colony thick 
incrusting. and fleshy. 
Botrylloides. Sarcolotrylloides. 
It is a difficult question to determine whether, in the ancestral history of the group, 
the modification of the systems or of the colon}’ took place first, whether, in fact. Scheme 
A or Scheme B most nearly represents the phylogeny of the family. 
Sarcobotrylloides. 
Botrylloides. j 
Ancestral 
Botrylloides. 
Polycyclus. 
Botryllus. 
\y 
Ancestral 
Botryllus. 
Ancestral Botrtllid^. 
Scheme A. 
Polycyclus. 
Botryllus. 
Sarcohotrylloides. 
Botrylloides. 
Ancestral 
Sarcohotrylloides. 
Ancestral 
Botrylloides. 
Ancestral Botryllid.'E. 
Scheme B. 
It must also be left doubtful whether in Scheme A the thin [Botryllus) or the thick 
[Polycyclus) condition of the colony was the more primitive ; and whether, in Scheme B, 
the irregular [Botrylloides) or the regular [Botryllus) condition of the system was the 
more primitive. Probably both in the case of the colony and of the system the extreme 
forms which we see at the present day diverged from intermediate forms ; and the 
ancestral Botryllus of Scheme A formed a moderately thick colony, which might become 
reduced into a Botryllus or increased to form a Polycyclus, while the ancestral Botrylloides 
of Scheme B had an irregular arrangement of Ascidiozooids, which became modified on the 
one hand into regular circular systems as in Botryllus, and on the other hand into long 
branching lines as in Botrylloides (see Fig. 8, D and E, p. 39). 
