8o 



ASCIDIANS 



CHAP. 



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 liable 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 



FIG. 44. Colonies of Compound Ascidians (nat. size). A, Colella quoyi, Hrdn. 

 Antarct. ; B, Leptoclinum neglectum, Hrdn. ; C, Pharynrjodictyon mirabile, Hrdn. 

 Southern Ocean ; D, Botryllus sMosseri, 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 Qlavelinidae, 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.c). Such 



