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 number 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- 
Fic. 246.—Diagrammatic longitudinal vertical 
section through the anterior end of Balano- 
glossus minutus. Slightly altered, after 
Bateson. 
1. Dorsal nerve cord. 
iw) 
Central nervous system (7.e. in the coliar 
region). 
Cord running from the central nervous system 
to the skin. 
. Ring of nervous tissue round the collar. 
Ventral nerve cord. 
. Dorsal blood-vessel. 
. Heart. 
. Ventral blood-vessel. 
co ST Os OU > 
. Proboscis gland. 
. Notochord. 
. Cavity of proboscis. 
. Cavity of collar. 
Mouth. 
Gill-slits. 
. Oesophagus. 
eS Se eS eR ee 
oO rF OF De CO CO 
walled pulsating vesicle lying in the base of the proboscis, just 
dorsal to the anterior end of the notochord. The heart is 
