CHORDATA 447 
do not correspond with so many gill-slits, but are rather sub- 
divisions of a single original pair. 
The Urochordata are divided into three orders: (i.) Larv- 
ACEA, (ii.) THALIACEA, and (i11.) ASCIDIACEA. 
Order 1. LARVACEA. 
The Larvacea include but four genera, the best-known of 
which is the genus Appendicularia. The members of the 
group are of extreme interest, since they have undergone no 
degeneration, but retain those features of the active larval 
condition which are lost in the other members of the group. 
Appendicularia, like the other Larvacea, is a minute free- 
Fic. 261.—Semi-diagrammatic view of Appendicularia from the right. 
After Herdman. 
a, Anus. ot. Otocyst. 
at, One of the atrial apertures. ov, Ovary. 
app. Tail. pp. Peripharyngeal band. 
brs. Branchial sac. ng. Cerebral ganglion. 
br. Branchial aperture. ng’. Caudal ganglion. 
dt, Dorsal tubercle. ng’. Enlargement of nerve cord in tail. 
end, Endostyle. so. Sense organ (tactile) on lower lip. 
h. Heart. sg. Ciliated aperture in pharynx. 
i. Intestine. st. Stomach. 
m. Muscle band in tail. tes. Testes. 
n. Nerve cord in body. u. Notochord. 
n'. Nerve cord in tail. uw’. Its cut end. 
oe. Oesophagus. 
swimming organism, found near the 
parts of the ocean. Its body is divided into a short trunk and 
surface of the water in all 
