80 ASCIDIANS : CHAP. 
se 
shoe remains small, while the horns become long and much coiled 
so as to constitute two prominent spirals (8, 9,10). In other 
exceptional forms again the curved slit becomes straightened out, 
undulating (11), irregularly bent (12 and 13), elaborately folded 
(14 and 15), or broken up into pieces (16), so that there come 
to be several or even a large number (17 and 18) of minute 
openings in place of the original single aperture. 
It cannot be said that any form of dorsal tubercle is charac- 
teristic of any of the families or genera of Ascidians, and in the 
case of some species the organ is lable to great individual 
variation ; but still in most species there is found to be a char- 
acteristic shape or appearance of tubercle which is a useful 
diagnostic feature. 
Sub-Order 2. Ascidiae Compositae. 
Fixed Ascidians which reproduce by gemmation so as to form 
colonies (Fig. 44) in which the ascidiozooids are buried in a 
Fic. 44.— Colonies of Compound Ascidians (nat. size). ..A, Colella quoyi, Hrdn. 
Antaret. ; B, Leptoclinwm neglectum, Hrdn. ; C, Pharyngodictyon mirabile, Hrdn. 
Southern Ocean ; D, Botryllus schlosseri, Sav. Europe. (After Herdman.) 
common investing mass (Fig. 45) and have no separate tests— 
hence “Synascidiae,” a name they often receive from foreign writers. 
This is probably a somewhat artificial assemblage formed of 
those two or three groups of Ascidians which produce colonies, 
in which the ascidiozooids are so intimately united that they 
possess a common test or investing mass. This is the only char- 
acter which distinguishes them from the Clavelinidae, but 
the property of reproducing by gemmation separates them from 
the rest of the Ascidiae Simplices. In some cases the atrial 
apertures of several neighbouring ascidiozooids join to open to 
the exterior by a common cloacal aperture (Fig. 45, ¢.c). Such 
