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 endosbyle. This organ secretes the 

 mucns 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 Qgg 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 tlie tail G. 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 ; 

 c7t, notochord or primitive backbone. 



(From Gegenbaur's ' Elements of Comparative Anatomy.') 



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 swinuning 

 freely for a few hours, the larva settles down head foremost and 



