Final answer:
The slightly differentiated stem cell in sponges is called an amoebocyte. Amoebocytes have the ability to differentiate into other cell types within the sponge and play various roles in nutrient delivery and reproduction.
Step-by-step explanation:
The slightly differentiated stem cell in sponges is called an amoebocyte. Amoebocytes are derived from stem-cell-like archaeocytes and have the ability to differentiate into other cell types within the sponge, such as collenocytes, lophocytes, sclerocytes, and spongocytes. They play various roles in delivering nutrients, producing reproductive cells, and creating skeletal structures. Amoebocytes are important for maintaining the functionality of sponges.