CHORD AT A 



437 



A constant current of water, maintained by the action of 

 the cilia lining the pharynx, enters the mouth and passes out 

 through the slits in the branchial sac into the atrial cavity, 

 and out at the atrial pore, carrying with it the waste matter 



^ &ric/ 



^ 'OV 



Fig. 253. — Diagrammatic dissection 

 of Ascidia mentula to show the 

 anatomy of a simple Ascidian. 

 After Herdman. 



at. Atrial aperture. 



br. Branchial aperture, 



a. Anus. 



irs. Branchial sac. 



dl. Dorsal lamina. 



dt Dorsal tubercle. 



end. Endostyle. 



h. Heart. 



i. Intestine. 



m. Mantle. 



ng. Nerve ganglion. 



ce. Oesophagus. 



ce.a. Oesophageal aperture. 



0(1. Ovary. 



pbr. Peribranchial cavity. 



r. Rectum. 



si. Stomach. 



f. Test. 



in. Tentacles. 



vd. Vas deferens. 



ngl. Subneural gland. 



from the alimentary canal and also the generative products 

 when ripe. 



A little distance within the oral opening is situated a ring 

 of tentacles, which project across the mouth and possibly 

 function as a filter; the area between this ring of tentacles 

 and the branchial sac is termed the prebranchial zone. The 

 branchial sac is a large space with perforated walls; it is 

 attached to the mantle or. body- wall along the median ventral 

 line, and the large ventral blood-vessel, which communicates 

 behind directly with the heart, is situated along this liue of 

 attachment. The rest of the branchial sac is separated from 

 the mantle by the peribranchial or atrial chamber, which 

 opens to the exterior by the atrial pore. The rectum and 



