THE TUNICATA. 



515 



tures are at opposite ends of the longest diameter of the 

 body ; and, in the two latter, locomotion is effected by the 

 contraction of transverse muscular bands, which drives the 



Fig. 148.— Phallusia mentula.— The test is removed, and hardly more of the animal 

 represented, than would, be seen in a longitudinal section: a, oral aperture; b, 

 ganglion; c, circlet, of tentacles ; d, branchial sac— the three rows of apertures in 

 its upper part indicate, but do not represent, the stigmata; e, the languets ; /, the 

 oesophageal opening ; g, the stomach ; h, the intestine ; i, the anus ; k, theatrium; 

 I, the atrial aperture ; m, the endostyle ; n, the heart. 



water out of the one aperture or the other, and causes the 

 body to be propelled in the opposite direction. 



When one of the simple fixed Ascidians, such as a Phal- 

 lusia (Fig. 148) or a Cynthia, is laid open by a section car- 



