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 cesophagus into the 
stomach. The pharyngeal walls between the 
stigmata carry blood-vessels, and the constant 
stream of water over them serves to erate the blood. 
Thus the pharynx of the Ascidian, like the ctenidia of 
Anodonta, functions for alimentation as well 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 Axodonta, ctenidia being originally respiratory 
organs. 
Respiratory. 
Fig. 204.—OBLIQUE SECTION THROUGH AN ASCIDIAN. (Ad nat.) 
Brain. 
Subneural Gland. 
Longitudinal 
Muscle. 
Dorsal Blood-vessel. 
Dorsal Lamina. 
Pharyngeal 
Wall. 
Pharyngeal 
Test Cavity. 
est. 
Endostyle. 
Heart. 
The circular muscles are scattered throughout the body- 
wall, but mainly concentrated as large sphincter rings around 
Muscular, ‘the, 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 and a ventral vessel, connected by vessels 
and sinuses. The dorsal vessel runs above the epibranchial 
