XII 



PHYLUM CHORDATA 



3'5 



tion of sphincters of muscular fibres which surround them. At the same 

 time the walls *of the body contract, streams of water are forced out 

 through the apertures, and the bulk becomes considerably reduced. 



or.ap 



at rap 



FIG. 198. Dissection of Ascidia from the right-hand side. The greater part of the 

 test and mantle has been removed from that side so as to bring into view the 

 relations of these layers and of the internal cavities and the course of the aljmen- 

 tary canal, etc. an, anus; atr. ap, atrial aperture; end, endostyle; gon, gonad; 

 gonod, gonoduct; hyp, hypophysis; hyp.d, duct of hypophysis; mant. mantle; 

 ne.gn, nerve-ganglion; aes. ap, aperture of oesophagus; or.ap, oral aperture; 

 ph, pharynx; stom, stomach; tent, tentacles; test, test. (After Herdman.) 



The outer layer of the body-wall is composed of a tough translucent 

 substance forming a thick test or tunic (Fig. 198, test}. This proves, 



