题目

        Zanzibar，Tanzania--Hundreds of dead dolphins were washed up on Friday along the shore of a popular tourist place on Tanzania’s northern coast．Scientists have ruled out poisoning．     It was not immediately clear what killed the 400 dolphins，whose dead bodies lied along a 4-kilometer coast of Nungwi，said Narriman Jidawi，a marine biologist at the Institute of Marine Science in Zanzibar．But the bottleneck dolphins，which live in deep offshore waters，had empty stomachs，meaning they could have got lost and had been swimming for some time to reorient themselves水螅的体壁不包括（　　）A．中胶层B．外胚层C．内胚层D．中胚层