Histology of the ascidian Botryllus schlosseri tunic: in particular, the test cells

From the tadpole stage we have examined the tunic of Botryllus schlosseri and particularly the histology of the test cells. Three distinct types of cells were recognized. The first, the fusiform, elongated and without filopodia, mere distributed along the vessel walls. The second, the fibrocytes, had granular cytoplasm and short pseudopodia. The third, the vacuolated, had cytoplasm, much vacuolated, and long filopodia. Our findings indicate that the vacuolated cells are the blood morula cells transformed after having migrated through the epidermis or endothelium into the tunic.