PHYLUM CHORDATA 



317 



simple muscular sac, situated near the stomach in a pericardium forming 

 part of the primitive ccelome. Its mode of pulsation is very remarkable. 



lent 



leal 



atom 



co.rci.'viac 



FIG. 186. Ascidia, diagram of longitudinal section from the left-hand side, the test 

 and mantle removed, atr. caz>. atrial cavity ; atr. siph. atrial siphon ; br. car. 

 branchio-cardiac vessel ; card. vise, cardio-visceral vessel ; gonod. gonoduct ; ht. 

 heart ; hyp. hypophysis ; mant. mantle ; , gn. nerve-ganglion ; ces. oesophagus ; 

 ov. ovary ; rect. rectum ; stigm. stigmata ; stom. stomach ; tent, tentacles ; test. 

 test ; tr. v. transverse vessel ; vent. v. ventral vessel ; vise. br. viscero-branchial 

 vessel. (After Herdman.) 



The contractions are of a peristaltic character, and follow one another 

 from one end of the heart to the other for a certain time ; then follows 



