II ASCIDIAE SIMPLICES—-SPECIFIC CHARACTERS 79: 
of which are—JMolgula oculata, Forbes, thickly covered with gravel 
or broken shells, and forming an ovate mass as large as a walnut ; 
and Hugyra glutinans, Moller, a smaller more globular body, the 
size of an acorn, and covered with fine sand, except at one circular 
area near the posterior end, where the leaden grey test shows 
through. Both these species are obtained by dredging in from 
10 to 50 fathoms, and lie freely on the bottom. <A rather rarer 
littoral species Molgula citrina, Hancock, found on some parts of 
our coast (e.g.1n the Firth of Forth, at Arran, and at Port Erin), 
is exceptional in having the test free from sand, and in being 
fixed like an Ascidia, generally to the lower surfaces of large 
stones near low tide. 
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. 
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Fic. 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 cerebri- 
formis ; 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. Poly- 
carpa 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 horse- 
