ASCIDIA. 



291 



covering the inner surface of the pharynx cause the currents 

 of water already referred to ; but, whilst the water itself is 

 carried through the stigmata into the atrium and thence to 

 the exterior, the food-particles become entangled in the 

 mucus and are transferred through the oesophagus into the 

 „ stomach. The pharyngeal walls between the 



stigmata carry blood-vessels, and the constant 

 stream of water over them serves to aerate the blood. 



Thus the pharynx of the Ascidian, like the ctenidia of 

 Anodonta^ functions for alimentation as viell as respiration, 

 though it should be carefully noted that in the former 

 the alimentation is the original primitive function, the 

 respiration being acquired later ; whereas the reverse 

 holds in Anodonta^ ctenidia being originally respiratory 

 ors^ans. 



Fig. 204. — Oblique Section through an Ascidian. {Ad nat.) 



Brain. 



Subneural Gl; 



Dorsal Blood-vessel. 



Dorsal Lamina. 



Atrium 



Pharyngeal 

 Cavity. 



Endostyle. 



Heart, 



The circular muscles are scattered throughout the body- 

 wall, but mainly concentrated as large sphincter rings around 

 Muscular ^^ mouth and atriopore. Similarly the longi- 

 tudinal muscles are best developed in relation 

 to the two external apertures. The circular muscles serve 

 to close the apertures and the longitudinal to contract the 

 whole body. 



The ccelom is not present as a definite perivisceral space, 

 but the blood-vascular system is not difficult to follow. It 

 consists of a dorsal d.rvdi a veftfral wq?>?>q\ connected by vessels 

 and sinuses. The dorsal vessel runs above the epibranchial 



