Sensory receptors and surface ultrastructure of trypanorhynch cestodes

The scolices of six different trypanorhynch species - Heteronybelinia alloi otica (Dollfus, 1960), Pseudolacistorhynchus noodti Palm, 1995, Otobothrium cysticum (Mayer, 1842), O. penetrans Linton, 1907, Poecilancistrum caryoph yllum (Diesing, 1850), and Prochristianella hispida (Linton, 1890)- were ex amined for surface morphology and the occurrence of sensory receptors. Fila mentous microtriches with different internal ultrastructural features were found. Acerosate, hooklike, and spiniform microtriches were detected on the surface of the tentaculariid H. alloiotica. Their internal structure clear ly differed from that of pectinate microtriches observed in the other five trypanorhynch species lacking a basal and a junctional region. All pectinat e microtriches had the same general architecture, independent of the number of digitiform processes. All trypanorhynchs studied harbored ciliated sens ory receptors within the tegument. Even though sensory receptors were scarc e in H. alloiotica, they were more abundant in the lacistorhynchid P. noodt i and the otobothriids P. caryophyllum and O. penetrans, which exhibited tw o, six, and three kinds of receptors, respectively. Bothridial pits in O, p enetrans and O. cysticum were invaginations of the bothridial surface, bein g characterized by the lack of sensory receptors and the presence of charac teristic microtriches. These differed from other microtriches in that they were larger and had a base consisting of a widely enlarged matrix. The occu rrence of different kinds of microtriches and sensory receptors within tryp anorhynch cestodes is summarized, and the meaning of these surface structur es and of bothridial pits as characters within future trypanorhynch classif ication is emphasized.