SPECIFIC CHARACTERS AND DORSAL TUBERCLE.—The chief points in which the various genera and species of Simple Ascidians differ are the details of the branchial sac (see Figs. 22, 35, 36, 38, and 41), the condition of the tentacles (Fig. 37), the dorsal lamina or languets, and the dorsal tubercle, in addition to form, colour, and other external features. [Illustration: FIG. 43.—Various forms of dorsal tubercle in Simple Ascidians. 1. _Molgula pyriformis_; 2. _Forbesella tessellata_; 3. _Ascidia meridionalis_; 4. _Cynthia formosa_; 5. _Cynthia papietensis_; 6. _Ascidia challengeri_; 7. _Polycarpa tinctor_; 8. _Cynthia cerebriformis_; 9. _Ascopera gigantea_; 10. _Boltenia tuberculata_; 11. _Ascidia translucida_; 12. _Culeolus moseleyi_; 13. _Ascidia pyriformis_; 14. _Boltenia pachydermatina_; 15. _Microcosmus draschii_; 16. _Styela etheridgii_; 17. _Styela whiteleggii_; 18. _Polycarpa aurata_. (After Herdman.)] Fig. 43 shows some of the more remarkable forms of dorsal tubercle. Starting with a simple circular opening (1) surrounded by a thickened ciliated ring, the anterior border becomes pushed in to form a crescentic slit (2 and 3). The horns of the crescent then grow longer and may be turned in (4 and 5) or out (6 and 7), and so give rise to the many varieties of horse-shoe (such as 6), perhaps the commonest form of dorsal tubercle in Simple Ascidians. In many Cynthiidae the central part of the {80}horse-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 characteristic 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 characteristic 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 common investing mass (Fig. 45) and have no separate tests—hence "Synascidiae," a name they often receive from foreign writers. [Illustration: FIG. 44.—Colonies of Compound Ascidians (nat. size). A, _Colella quoyi_, Hrdn. Antarct.; B, _Leptoclinum neglectum_, Hrdn.; C, _Pharyngodictyon mirabile_, Hrdn. Southern Ocean; D, _Botryllus schlosseri_, Sav. Europe. (After Herdman.)]