64 ASCIDIANS—APPENDICULARIANS CHAP. 
two ventral apertures). The ventral heart is simple and tubular, 
and periodically reverses the direction of the blood-current. 
= ARVACEA 
THALIACEA > 
aA 
4 
Ascidiids 
Molgulids Ascidians 
Fic. 27.—Sketch of the chief kinds of Tunicata found in the sea. 
This Class is divided into three Orders:—The Appendicularians, 
the Ascidians, and the Salpians (see Fig. 27). 
Order I. Larvacea (Appendicularians). 
Free-swimming pelagic forms, in which the posterior part of 
the body takes the form of a large locomotory appendage, the 
