Sensory receptors and surface ultrastructure of trypanorhynch cestodes.

The scolices of six different trypanorhynch species--Heteronybelinia alloiotica (Dollfus, 1960), Pseudolacistorhynchus noodti Palm, 1995, Otobothrium cysticum (Mayer, 1842), O. penetrans Linton, 1907, Poecilancistrum caryophyllum (Diesing, 1850), and Prochristianella hispida (Linton, 1890)--were examined for surface morphology and the occurrence of sensory receptors. Filamentous microtriches with different internal ultrastructural features were found. Acerosate, hook-like, and spiniform microtriches were detected on the surface of the tentaculariid H. alloiotica. Their internal structure clearly differed from that of pectinate microtriches observed in the other five trypanorhynch species lacking a basal and a junctional region. All pectinate microtriches had the same general architecture, independent of the number of digitiform processes. All trypanorhynchs studied harbored ciliated sensory receptors within the tegument. Even though sensory receptors were scarce in H. alloiotica, they were more abundant in the lacistorhynchid P. noodti and the otobothriids P. caryophyllum and O. penetrans, which exhibited two, six, and three kinds of receptors, respectively. Bothridial pits in O. penetrans and O. cysticum were invaginations of the bothridial surface, being characterized by the lack of sensory receptors and the presence of characteristic microtriches. These differed from other microtriches in that they were larger and had a base consisting of a widely enlarged matrix. The occurrence of different kinds of microtriches and sensory receptors within trypanorhynch cestodes is summarized, and the meaning of these surface structures and of bothridial pits as characters within future trypanorhynch classification is emphasized.