TUNICATA. 87 
Passing backwards along the ventral edge of the branchial sac 
is a thick-lipped furrow, which appears like a rod in the thin-walled 
sac, and hence is called the endostyle. This organ secretes the 
mucus which is carried up by ciliary action to the circular groove 
in front of the branchial sac, and thence to the gullet along a fold 
or crest, termed the dorsal lamina, situated along the dorsal edge of 
the branchial sac. 
The gullet opens into a large stomach situated posteriorly on the 
left side of the branchial sac. The stomach opens into the intestine, 
which, after forming a loop, terminates in the anal orifice or vent 
opening into the atrial cavity. 
The tubular heart lies below the stomach, a remarkable feature 
in the circulation consisting in the periodic reversal of the blood 
current. An elongated nerve ganglion is situated between the 
branchial and atrial orifices. 
Ascidia mentula is hermaphrodite. The egg develops into a 
minute tadpole-like larva which swims about by means of its tail. 
Water entering by the mouth passes out through the gill-slits. A 
nerve-tube extending along the back and tail is swollen in front into 
a brain-vesicle ; and underneath the long nerve-tube behind the 
Ascidian Tadpole with part only of the tail 0. Magnified section. 
N, nervous system with enlarged brain in front and narrow spinal cord behind 
n ; N', cavity of brain ; 0, the single cerebral eye lying in the brain ; 
a, auditory organ ; K, pharynx ; d, intestines ; o, rudiment of mouth ; 
ch, notochord or primitive backbone. 
(From Gegenbaur's ' Elements of Comparative Anatomy. 1 ) 
brain is a stiff skeletal rod or axis — the notochord — which constitutes 
the rudiment of a backbone. Inside the brain are two unpaired sense 
organs, an eye and an organ of hearing (Fig. 5). After swimming 
freely for a few hours, the larva settles down head foremost and 
