Here's why:

* ER's role: The endoplasmic reticulum is a network of membranes found in eukaryotic cells. It plays various roles, including protein synthesis, lipid synthesis, and calcium storage.

* Sarcoplasmic reticulum's specialization: The sarcoplasmic reticulum is a specialized type of ER found in muscle cells. Its primary function is to regulate calcium levels, which are crucial for muscle contraction.

Therefore, the sarcoplasmic reticulum can be considered a specialized form of the endoplasmic reticulum, adapted for the specific needs of muscle cells.