EEPORT ON THE TIJNICATA. 
393 
which gave rise to most of the Compound Ascidians. Diazona diverged from the 
main line before the Ascidiozooids became completely imbedded in the common test, 
and just before the suppression of the internal longitudinal bars in the branchial sac 
took place, as these vessels are still present in Diazona, although absent in Clion- 
drostachys, the next genus which left the main axis (see fig. 13). In the line leading 
to Chondrostachys the basal part of the colony became enlarged and prolonged to form 
Leptoclinum. 1 
Eucaelium.- 
Pyrosorna. 
CoelocormiLS. 
e 
cs e 
o 
H. 
Distoma. 
Oxycorijnia.- 
G. 
Chondrostachys.- 
■Diazona. 
C. 
Fig. 13. — Diagram illustrating the phylogeny of the Distomidse, Coeloeormidae, Pyrosomida?, Diplosomidse, and 
Didemnidae. C. indicates the point where the ancestral Polyclinid® and Distomidse diverged. 
a peduncle, supporting the upper part in which the Ascidiozooids are placed. As in 
the case of Diazona, the Ascidiozooids are not completely imbedded in the test, but 
have their anterior ends partially free. In all the forms above this, however (see fig. 13), 
a true colony is formed by the complete union of the tests, so as to bury all the Ascidio- 
zooids in a common investing mass. 
At the point G. the main axis of the Distomidse divided into two lines of descent : 
(ZOOL. CHALL. EXP. — PART XXXVIII. — 1886.) Pp 60 
