Cephalic anatomy of a gnathostomatid nematode Echinocephalus sinensis, parasite of oysters and rays

Light, transmission, and scanning electron microscopy of both adult and third‐stage Echinocephalus sinensis shows that the head is characterized by a circular muscle layer in the headbulb wall, a highly unusual arrangement in nematodes. The basal region of cephalic spines is enclosed by all three cuticular zones but the distal region of its shaft is lined only by the cortical layer. The so‐called “ballonet‐cervical sac” system is actually formed by four modified cervical muscle cells of the circomyarian type, each with a liquid cytoplasmic matrix serving as a hydrostatic chamber. The headbulb is deflated by contraction of the circular wall muscle and six pairs of specialized longitudinal and oblique muscles. Two pairs of oblique somato‐oesophageal muscles also serve to shorten the oesophagus. Relaxation of the muscles and an anterior flow of fluid into the cephalic region of the cervical muscle cells inflate the headbulb. In adult worms, the trilobed pseudolabia are lined internally mostly by the oesophageal cuticle. Four radially arranged muscles help to dilate the buccal cavity and the pseudolabia can be retracted into the headbulb by two pairs of oblique muscles inserted at their base. The radial musculature at the anteriormost oesophagus has more abundant and tightly packed myofilaments than other regions. Four pitlike structures of unknown function are located near the base of the pseudolabium. In the third‐stage worm the pyriform pseudolabium is internally lined mostly by the body cuticle. Two rows of bulbous structures each with a central process are located on the headbulb a short distance from the pseudolabium. Two pairs of oblique buccal dilatory muscles help to dilate the oral opening and draw the pseudolabia towards the headbulb. Two bands of oblique myofilaments are present within the anterior‐most region of the oesophagus. The functional adaptation of the cephalic system in relation to the biology of the parasite is discussed.