Ultrastructure of presumed sensory receptors in the scolex of adult Proteocephalus exiguus (Cestoda, Proteocephalidea).

The ultrastructure of six type of putative sense receptors including three nonciliate (Types I-III) and three ciliate (Types IV-VI) types found in the tegument of the scolex of adult Proteocephalus exiguus has been described for the first time. Type I has a simple tegumental sensory ending, containing one electron-dense collar, circular septate desmosome, electron-lucent vesicles and microtubules. Type II is represented by a receptor with a large rootlet. Type III is a receptor with a very long cross-striated rootlet, with two electron-dense collars and septate junction. The ciliated receptors differ in the length of the cilium, in the number of electron-dense collars and in the shape of sensory bulb. A comparison of structural features of receptors in different systematic group within Platyhelminthes is made.