440 



ZOOLOGY 



and- form the posterior limit of the prebranchial zone (Fig. 



Fig. 255. — Transverse section through 

 the branchial region of a dona mtes- 

 tinaZis. After Ronle. 



1. Atrial chamber. 



2. Branchial chamber. 



3. Test. 



4. Mantle. 



8. Eectum. 



9. Oviduct. 



10. Vas deferens. 



11. Ciliated endostyle. 



6. Vessel passing from a branchial bar 12. Branchial slit, stigma. 



into the mantle. 13. Branchial bar connected with mantle. 



6. Ventral blood-vessel at base of endo- 14. Muscle fibres. 



style. 15. Dorsal lamina. 



7. Dorsal blood-vessel at base of dorsal 



lamina, 



254). • The anterior of these two ridges is the most pro- 

 minent ; it forms a simple circle ; the posterior is, however, 



Fig. 256. 



A. Part of branchial sac of 



Ascidia from inside. 



B. Transverse section of same, 



drawn to different scale. 



tr. Transverse vessel. 



d. Connecting duct. 

 hm. Horizontal membrane. 



il. Internal longitudinal bar. 



Iv. Fine longitudinal vessels. 

 p, p'. Papillae. 



sg. Stigmata. 



Ti-otiffh 



continuous with the dorsal lamina, a structure which runs 



