398 
THE VOYAGE OF H.M.S. CHALLENGER. 
The primitive Cynthiidee derived from the ancestral form J. (fig. 11, p. 388) acquired • 
longitudinal folds in the branchial sac and then divided into two series, the one leading 
to the Styelinse and the Polystyelidae, and the other giving rise to the remaining Cynthiidae 
9 ,nd the Molgulidse. In the primitive Styelinse the tentacles remained simple, as they 
were in the ancestral Simple Ascidians, and the number of folds in the branchial sac 
became limited to four on each side. The reproductive organs assumed the form of two 
or more hermaphrodite masses (called polycarps) attached to the inner surface of the 
mantle upon each side of the body. At the point K. (fig. 11, p, 388) these ancestral 
forms divided into two series, those leading to the true Styelinae on the one hand, and to 
the primitive Poly sty elides on the other. 
■« 
Ct! 
-Symplegma. 
-Botryllm 
-Polycyclus. 
-Sarcohotrylloid.es. 
-Botrylloides. 
-Polystyela. 
Thylacium- 
Choiizocormus. 
K. 
Fig. 15. — Diagram illustrating the phytogeny of the Polystyelidae and the Botryllidse, and their possible connection. 
K. indicates the point where the PolystyeMae diverged from the Styelinse. 
The genus Chorizocormus is an important transition form between the Styelinse and 
the Polystyelidae proper (see p. 345). It is probably the nearest form known to the 
ancestral Polystyelidae, and may therefore be placed (see fig. 15) on a short side branch 
springing from the axis of the Polystyelidae not far above the point K. 
Thylacium and Polystyela are derived from side branches between the ancestral 
Chorizocormus and the ancestral Synstyela or Goodsiria. They have the Ascidiozooids 
projecting somewhat above the general surface of the colony, and therefore bear much 
the same relation to the higher Polystyelidae that the genus Diazona does to the typical 
Distomidae. The axis of the Polystyelidae finally divides into two branches, allowing the 
two most highly evolved forms in the family, Synstyela and Goodsiria, to diverge in 
opposite directions (see fig. 15). In Synstyela the colony has become thin and incrusting, 
