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This work represents the first ultrastructural study of the tegument of a freshwater teleost-infecting aporocotylid blood fluke and the first detailed observations of the surface sensory receptors of any aporocotylid. The work was carried out on Sanguinicola inermis from the branchial arteries of the pike, Esox lucius. Light and scanning electron microscopy observations failed to show the presence of protruding surface spines in S. inermis. Transmission electron microscopy studies revealed, deep beneath the distal cytoplasm of the tegument, sporadic, membrane-bound sarcoplasmic processes, each containing a single spine with the typical crystalline structure of digenean spines. On rare occasions, the spine may protrude above the body surface, but in such cases the distal tegumental cytoplasm is extremely flattened. This finding supports a unique, deep origin of aporocotylid spines when compared to those of other digenean families. This character may well prove to be an autapomorphy of the Aporocotylidae, and the position of the base of aporocotylid spines within sarcoplasmic processes supports their origin as a muscle derivative. Unlike other digenean groups, the distal region of the cytoplasmic processes, connecting sunken tegumental perikarya with the syncytial tegumental cytoplasm, is strengthened by peripheral microtubules. Three types of ciliated and three types of non-ciliated sensory receptors were distinguished in the tegument of S. inermis. One of the distinctive features of the ciliated receptors is the unusual position of the basal region of the cilium within a tegumental outgrowth. Such a position has previously been described for certain receptors types in larval and adult schistosomatids and larval diplostomids, suggesting that this feature, in adult worms, might prove to be an apomorphic trait for some or all groups of blood flukes.

The intrauterine eggs of the pleurogenid trematode Brandesia turgida ( Brandes, 1888 ), exhibiting advanced stages of miracidial differentiation and fully formed, ciliated miracidia, were examined by means of transmission electron microscopy (TEM). Each embryonated egg is composed of a mature miracidium surrounded by a four-layered egg wall: (1) an outer, anucleate layer external to the eggshell, which forms a thick cocoon; (2) the operculate egg-shell; (3) a small remnant of the compact, granular cytoplasm of the outer embryonic envelope ( sensu stricto ); and (4) a relatively distinct cellular remnant of the inner embryonic envelope. Layers enveloping the egg apparently play an important role in the protection, metabolism and storage of nutritive reserves for the developing miracidium. The outer, anucleate layer, or cocoon, situated externally to the eggshell and composed of a transparent, electron-lucent substance with numerous dense, osmiophilic islands attached to its peripheral membrane, has never previously been seen in TEM studies of the eggs of parasitic platyhelminths. The origin, formation, functional ultrastructure and chemical composition of this peculiar layer remain enigmatic, although its function appears to be protective. The thick, electron-dense eggshell resembles that of other trematodes, exhibiting a characteristic fissure zone around the operculum. The very small, indistinct remnants of the outer embryonic envelope appear in the form of a very thin, compact, granular cytoplasm closely attached to the inner surface of the eggshell. Conversely, the inner embryonic envelope is frequently apparent at one or both poles of the developed egg as a syncytial envelope formed by the fusion of mesomeres. This envelope, even in eggs containing a fully formed miracidium, still has the features of a metabolically active layer with an energy storage capability. Lysosome-like structures observed in some eggs may be involved in the autolysis of the embryonic envelopes.