6 4 



ASCIDIANS APPENDICULARIANS 



CHAP. 



two ventral apertures). The ventral heart is simple and tubular, 

 and periodically reverses the direction of the blood-current. 



Fio. 27. Sketch of the ctiief kinds of Tunicata fouud 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 



