Ultrastructural studies of epidermis, sense receptors and sperm of Craspedella sp. and Didymorchis sp. (Platyhelminthes, Rhabdocoela)

Structure and evolution of the pharynx in the Temnocephalida (Platyhelminthes)

The structure of the pharynx was studied in the genera Didymorchis, Diceratocephala, Temnocephala, and Craspedella, using light microscopy, silver nitrate staining, and transmission electron microscopy. Based on these data, evolution of this organ within the Temnocephalida is analysed. In addition to the specific pattern of muscle fibres in the walls of the pharynx known from earlier studies, three other features characteristic of the pharynges of temnocephalids were found: (1) multisyncytial organization of the pharyngeal epithelium, (2) the presence of type 1 sensory receptors, and (3) muscle fibres incorporated into the epithelium of the pharynx. Features deduced to be characteristic of the pharynx in primitive temnocephalids are (i) the presence of three types of sensory receptors with specific morphologies and locations in the pharynx, (ii) the presence of a bundle of muscle fibres that traverses the epithelium surrounding the anterior margin of the pharynx, (iii) characteristic lamellation of the epithelium of the pharynx proper, and (iv) a narrow syncytium nesting the openings of the pharyngeal glands and forming deep invaginations into which the gland ducts open. The most prominent tendencies in the evolution of the pharynx are reinforcement of the pharyngeal musculature (related to the mode of feeding) and reduction in the number of syncytia.

Peripheral sensilla of some lower invertebrates: the Platyhelminthes and Nematoda

The flatworms (Platyhelminthes) and the round worms (Nematoda) are phyla exhibiting strikingly different levels of cellular organization. In both, sensilla are composed of the endings of sensory dendrites intercalated into their epidermis. In flatworms, sensilla that penetrate the syncytial epidermis bear sensory processes derived from cilia. In free-living species, the sensory processes more closely resemble motile cilia, while in parasites, greater deviations occur from the classical cilium pattern. Estimates of the function of the various sensilla have been largely arbitrary, and remain based on ultrastructural features. Sensilla in round worms lie below or within a heavy secreted cuticle. Two glia-like cell types occur. The socket cell mediates contact with cuticle and is responsible for cuticular modifications essential for operation of the sensillum. The sheath cell forms a receptor cavity around the sensory processes and regulates its environment. Sensory processes vary greatly from the classical cilium pattern. Absence of a basal body, but preservation of a ciliary necklace, suggests that the latter has a primary importance in sensory transduction. Estimates of function are based largely on ultrastructural features and analogies to arthropod sensilla. Genetic studies with the free-living nematode Caenorhabditis are beginning to demonstrate details of function and development. Speculations on the roles of basal bodies, rootlets, and vesicles and on the significance of recessed sensilla are given.