Ill 



ASCIDIAE SIMPLICES SPECIFIC CHARACTERS 



79 



of which are Molgula oculata, Forbes, thickly covered with gravel 

 or broken shells, and forming an ovate mass as large as a walnut ;. 

 and Eugyra 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 30 fathoms, and lie freely on the bottom. A rather rarer 

 littoral species Molgula citrina, Hancock, found on some parts of 

 our coast (e.g. in 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. 



FIG. 43. Various forms of dorsal tubercle in Simple Ascidians. 1. Molgula pyrij'm- mis . 

 2. Forbesella tessellata ; 3. Ascidia meridionalis ; 4. Cynthia fornwsa ; 5. Cynthia 

 papietensis ; 6. Ascidia challenyeri ; 7. Polycarpa tinctor ; 8. Cynthia cerebri- 

 formis; 9. Ascopera yigantea : 10. Boltenia tuberculata ; 11. A scidia translucida ; 

 12. Culeolus moseleyi ; 13. Ascidia pyri/armis ; 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- 



