390 
THE VOYAGE OF H.M.S. CHALLENGEE. 
thorax, the abdomen, and the post-abdomen. Large colonies were formed, and the 
Ascidiozooids (produced by gemmation from the post-abdomen) composing the colony 
were so closely placed that their tests became united to form a continuous investing mass. 
On account of several young Ascidiozooids being usually produced from a single older 
Ascidiozooid in the colony, a more or less regular grouping into systems naturally took 
place, and then the atrial apertures of the various Ascidiozooids in a system coalesced to 
e 
s 
s 
s 
,s 
si 
P 
'G 
g 
1 
§ 
E. 
Atopocjaster.- 
Pharyngodictyon, - 
D. 
e 
G 
S 
6 
F. 
-Polyclinum. 
-Tylobranchion. 
-C. 
-Sigillina. 
-Polydinoid'es. 
Fio. 12. — Diagram illustrating the pbylogeny of the Polyclinidee. 0. indicates the point where the ancestral 
Polyclinidpe and Distomidas diverged. 
form a centrally placed common cloacal aperture. The branchial sac also underwent a 
gradual degeneration, resulting in the complete disappearance of the system of internal 
longitudinal bars inherited from the ancestral forms at B. (see fig. 11). 
The central axis of the Polyclinidee, extending from C. to D. (see fig, 12), was composed 
of a series of ancestral forms in which these and some other less important changes were 
gradually being effected, but from this axis a few short side branches were given off at 
