Sea anemones belonging to the genera Adamsia and Stylobates have a remarkable symbiotic relationship with hermit crabs. These symbiotic sea anemones produce a shell-like structure, called a “carcinoecium,” that covers and extends over the gastropod shell of the host hermit crab as hermit crabs grow. This structure has been described as “chitinous carcinoecium” or “chitinous coating.” A previous study investigated carcinoecia of Stylobates aeneus, the results of which indicated that it contained at least 1.7% chitin, while the remaining components were unidentified. Moreover, the microscopic structure of a carcinoecium still remains to be detailed. We, therefore, conducted detailed observations using a stereoscopic microscope and scanning electron microscope (SEM) and the analyses of the chemical composition of carcinoecia produced by Stylobates sp. (apparently conspecific with Isadamsia sp. “J” reported in Uchida and Soyama, 2001) associated with a pagurid hermit crab Pagurodofleinia doederleini collected in the south of the Shima Peninsula, Mie, Honshu Island, Japan at a depth of 294–306 m. Our results indicate that carcinoecia of Stylobates sp. contain HCl-soluble components (13%), NaOH-soluble components (38%), chitin (11%) and unidentified remnants (39%). Additionally, our observations show that Stylobates sp. incorporates dark- and white-colored particles that could be sand and/or mud into the carcinoecium.