Comparative studies on the morphology, histology, and histochemistry of metacestodes (Hymenolepididae, Dilepididae and Dipylidiidae)

Histological and histochemical methods were used for the studies on 12 species of metacestodes of the genera Coronacanthus, Diorchis, Diplopylidium, Fimbriaria, Hymenolepis, Microsomacanthus, Rodentotaenia, and Triodontolepis. Two basic types of larvae, cysticercoid and cercoscolex, were found. The cyst structure is identical or similar in the cysticercoids using the same group of intermediate hosts (gammarus, planktonic crustaceans, coprophagous beetles, and terrestrial snails). The morphological structure of scolex and neck is the same as in adult cestodes. The length and number of hooks are definitive in morphologically differentiated metacestodes, while the scolex, rostellum and suckers grow still inside the definitive host. The excretory canals open separately at the end of neck in the cysticercoids and in a common opening at the body end of larva in the cercoscolex. Calcareous corpuscles are smaller and more numerous in the scolex and neck than in the cyst wall. Acid mucosubstances prevail in the tissues at the early stages of development. In morphologically differentiated metacestodes, the amount of neutral mucosubstances and glycogen increases. In ageing larvae, the amount of neutral mucosubstances and glycogen gradually decreases and later also decreases the amount of acid mucosubstances, proteins and lipids.