ATLAS OF BIOLOGY. 



s.n. Siphonal nerve. 



at. Stomach. 



tc. Tactile organs of larva. 



ty. Typhlosole. 



u. Umbo. 



v. Ventricle. 



va'. Right auriculo-ventricular valve. 



va". Left auriculo-ventricular valve. 



v.c. Central blood sinus. 



v.m. Vitelline membrane. 



vs. Visceral branch of anterior aorta. 



