394 
THE VOYAGE OF H.M.S. GHALLENGER. 
a smaller branch leading to the genera Oxycorynia, Colella, and Distaplia, and 
a more important one, which has given rise to all the remaining forms (see fig. 13). 
In the ancestral forms of Oxycorynia, as in those of Chondrostachys, a peduncle has been 
formed, while the upper part of the colony forms an enlarged mass; the head, in which 
the short-bodied Ascidiozooids are imbedded. 
The Ascidiozooids probably underwent very little change in form after they diverged 
from the ancestral Polyclinidse at the point C. (fig. 11), and no antero-posterior elonga- 
tion like that of the Polyclinidje has taken place. In the genera Colella and Distaplia 
a remarkable modification of the peribranchial cavity has been effected, resulting in the 
formation of a large incubatory pouch in which the embryos undergo their development 
(see p. 89). 
The chief branch springing from the point G. (fig. 13) gave rise, with very little 
change, to the well-known genus Distoma, and the allied form Cystodytes. This latter 
genus is similar to Distoma in most respects, but differs from it in having a remarkably 
modified test, in which discoid calcareous spicules are produced (see p. 135). 
From the ancestors of Distoma a series of forms arose in which, while the general 
characters of the Distomidse were preserved, some important changes were effected in the 
test and in the reproductive organs. The test cells acquired the property of producing 
spherical or stellate calcareous spicules, while the vas deferens assumed gradually the 
spirally coiled form which is so characteristic of the Didemnidse (see p. 254). This 
ancestral line divided at the point H. into two branches, one leading with comparatively 
little change to the Didemnidse and the Diplosomidae, and the other producing the remark- 
able Ccelocormus, and eventually Pyrosoma (see fig. 13). In this second line diverging 
from H. the ancestral condition of the male reproductive organs found in the Distomidse 
was retained, along with the partially coiled arrangement of the vas deferens which was 
present in FI., and which afterwards became emphasised in the Didemnidse. At the same 
time the colony became detached, and its upper surface sank in so as to produce an 
axial cavity, the lining of which is really morphologically a part of the outer surface 
of the colony (see p. 318), thus giving rise to the genus Ccelocormus (fig. 14, B). 
The value of Ccelocormus as a transition form between the ordinary Compound 
Ascidian colony, such as a species of Distoma (fig. 14, A), and the pelagic Pyrosoma 
(fig. 14, C) has already been pointed out in the systematic part of this Eeport (see p. 319). 
Pyrosoma has probably descended from an ancestral form, allied to Ccelocormus, by 
slight changes in shape, resulting in the formation of an elongated hollow cylinder, and 
by a modification in the relations of the Ascidiozooids whereby they came to open 
independently into the large axial cavity, which is thus virtually converted into a huge 
common cloacal cavity (see fig. 14, C.). The colony is free-swimming, and the Ascidio- 
zooids have acquired light-producing organs placed laterally on their anterior ends in the 
positions occupied by the masses of pigment cells in Polycyclus jeffreysi (see p. 68). 
