Isabel Fernández

ULTRASTRUCTURAL OBSERVATIONS ON THE PHORONID NERVOUS SYSTEM

Standard transmission electron microscopy (TEM) techniques have been applied to three species of phoronids to describe cytological features of their nervous systems. Four types of neurons have been distinguished by morphological and ultrastructural criteria, mainly involving their kinds of vesicles. Glial cells with characteristic polymorphic dense granules are abundant in the plexus, with possible roles in neuronal nourishment, mechanical support, and neurosecretion. Synaptic contacts and neuromuscular junctions across the connective tissue are described. The different structural features of the trunk giant axons in the three species mainly concern the enveloping sheath, and are described and discussed. Data on the innervation of internal organs, such as gut and nephridia, are also given. A detailed description of the structure, arrangement, and relationships of tentacular sensory cells, probably mechanoreceptors, is included, with a tentative hypothesis concerning the functions of these cells.

Ultrastructure of the midgut and hindgut of Derocheilocaris remanei (Crustacea, Mystacocarida)

ABSTRACT

Ultrastructure of the labrum and foregut of Derocheilocaris remanei (Crustacea, Mystacocarida)

The cuticle‐lined foregut of Derocheilocaris remanei consists of the mouth with its associated labrum, and an undifferentiated esophagus. It is separated from the midgut by an esophageal valve. The labrum is a conspicuous structure moved by five pairs of muscles (four dorsoventral and one longitudinal). Four pairs of subcuticular glands open to its inner face forming two longitudinal, lateral rows of cuticular pores. Each secretory unit is composed of a glandular component (with one or two secretory cells), a neck cell, and a duct cell. In addition, a single gland cell opens mesially into the buccal cavity.

Acrocoelus glossobalani gen. nov. et sp. nov., a protistan flagellate from the gut of the enteropneust Glossabalanus minutus

Summary The ultrastructure of a new flagellate parasite, Acrocoelus glossobalani gen. nov. et sp. nov. is described. The fusiform organism shows an apical anterior concavity, a longitudinal groove, and two anterior flagella directed backward and arising from parallel basal bodies in a conspicuous flagellar pocket. The parasite wall is a trilaminate pellicle with subpellicular microtubules and a micropore. The cytoplasm contains a single, branched mitochondrion with tubular cristae, a supranuclear Golgi apparatus and large amounts of glycan granules. The nucleus contains a conspicuous nucleolus and distinct areas of heterochromatin. Three kinds of secretory organelles appear: type I, round and structurally complex, probably extrusomes; types II and III, rhoptrie- and microneme-like organelles, respectively. Based on the whole set of ultrastructural features of A. glossobalani , we cannot include this organism into any known genus, nor into any described suprageneric taxon, and consequently a new genus is created and classified as incertae sedis in the kingdom Protista.

The ultrastructural identity of Protoentospora ptychoderae Sun, 1910 (Apicomplexa, Coccidia), a parasite of the body cavity of the enteropneust Glossobalanus minutus (Kowalewsky, 1866): Growing trophozoites.

The ultrastructural organization of Protoentospora ptychoderaeSun, 1910 from the coelom of the enteropneust Glossobalanus minutus (Kowalewsky, 1866) is presented. The parasites, identified as growing trophozoites without parasitophorous vacuole, are typical Apicomplexa with a trilaminate pellicle perforated by a micropore. The apical structures which characterized the infective forms of this group (apical rings, conoid, rhoptries, micronemes and subpellicular microtubules) gradually disappear throughout the trophozoite development. The cytoplasm contains mitochondria with vesicular cristae, a well developed Golgi apparatus, free ribosomes, smooth and rough endoplasmic reticulum, and large stores of amylopectin and lipids. The nucleus has a prominent, eccentric nucleolus. The systematic position of Protoentospora is discussed from the present data and the available literature.

Ultrastructural observations on merogony of Protoentospora ptychoderae Sun, 1910 (Apicomplexa, Coccidia) from the enteropneust Glossobalanus minutus (Kowalewsky, 1866)

The ultrastructure of the merogony of Protoentospora ptychoderae Sun, 1910 from the coelom of the enteropneust Glossobalanus minutus (Kowalewsky, 1866) is described. Trophic forms dedifferentiated into meronts by losing components of both the apical complex and the pellicle. Prior to merozoite formation, multinucleate meronts showed a discontinuous pellicle inner membrane complex. Merozoites formed by ectomerogony or external budding. In daughter merozoites the subpellicular microtubules were the first apical complex structures that were differentiated; rhoptries, conoid and polar ring differentiated later, and lastly micronemes appeared. The new merozoite had a trilaminate pellicle with a micropore, and showed approximately 60 subpellicular microtubules, a short conoid, at least five rhoptries, one paranuclear Golgi apparatus and one nucleus including one nucleolus. No parasitophorous vacuole was found at any stage of merozoite development. The organellogenesis events are discussed and compared with already known data from other Coccidia.

Penetration into the gut cells of an enteropneust by the flagellate Acrocoelus glossobalani Fernández et al., 1999

Summary The penetration process into the gut cells of the enteropneust Glossobalanus minutus by the protistan flagellate Acrocoelus glossobalani is investigated by transmission electron microscopy. This organism is a fusiform cell that contacts the host cell by the edges of its apical concavity. At the entry site, the host cell develops a dense band parallel to and below the host plasma membrane. This membrane seems to be ruptured at the point of contact by substances secreted by apical organelles of the parasite resembling rhoptries and micronemes of apicomplexans. As Acrocoelus enters the host cell, the host develops pseudopodia-like projections which surround and finally enclose the parasite into a membrane-bound parasitophorous vacuole. At this stage, the host cell membrane remains disrupted at the initial site of contact with the parasite, and the host dense band lacks its initial substructure. It is suggested that Acrocoelus glossobalani enters the host cell by an active mechanism initiated and carried out by the parasite, but the host cell contributes to its engulfment into the parasitophorous vacuole.