428 



ZOOLOGY 



alimentary canal to the exterior. The slits are arranged 

 obliquely, so that in a single transverse section it is impossible 

 to see the whole course of any one. In the section (Fig. 245) 

 the gill-slit has been drawn diagrammatically. 



The posterior edge of the collar projects backward over a 

 few of the anterior gill-slits, the niunber varying in different 

 species. This fold is termed the opercular fold, and the recess 

 covered in by it has been regarded as a rudimentary atrial 

 cavity. 



The vascular system comprises a dorsal vessel which 

 extends from the tail to the heart. The latter is a thin- 



FlG. 246. — Diagrammatic longitudinal vertical 

 section through the anterior end of Balano- 

 glossus minid'us. Slightly altered, after 

 Bateson. 



10. 

 11. 

 12. 

 13. 

 14. 

 15. 



Dor.sal nerve cord. 



Central nervous system {i.e. in the collar 



region). 

 Cord running from the central nervous system 



to the skin. 

 King of nervous tissue round the collar. 

 Ventral nerve cord. 

 Dorsal blood-vessel. 

 Heart. 



Ventral blood-vessel. 

 Proboscis gland. 

 Notochord. 

 Cavity of proboscis. 

 Cavity of collar. 

 Mouth. 

 Gill-slits. 



walled pulsating vesicle lying in the base of the proboscis, just 

 dorsal to the anterior end of the notochord. The heart is 



