Morphological and anatomic observation of Polydora brevipalpa in Patinopecten yessoensis

The morphology and histological structures of Polydora brevipalpa were studied by living observation, histological staining technique and scanning electron microscopy (SEM). The results show that: 1) The live worm was translucent and light brown, with black pigmentation on the palps. Prostomium was round, and four eyes were in trapezoidal arrangement. The setae on Chaetiger 5 were modified and enlarged; major spines of Chaetiger 5 were falcate, displaying a lateral tooth. Branchiae began from Chaetiger 7, and blood vessels flowed in the middle of the body and branchiae. Pygidium was disc-like with a dorsal incision. 2) The middle of the palps had food groove,where hada large number of cilia. Occipital tentacle was absent. Beginning from Chaetiger 7, the dorsal bands of cilia extended across the dorsum, between each pair of branchiae. The neurochaetae were replaced by bidentate hooded hooks. 3) The body structure of P.brevipalpa was mainly composed of skin, muscle layer and digestive system. A lot of gland cells were concentrated in cuticular layers. The muscle system was highly developed and oblique muscle appeared. The villi on Epithelial cells of the digestive tube appeared densely and regularly which formed obvious striated border. Each segment had a pair of renal tubular, a pair of lateral vessel and a pair of glandular pouch. It is concluded that the boring mechansm of P.brevipalpa resulted from mechanical abrasion by modified chitinous setae on Setiger 5 and chemical action by a secretion.