Eggshell formation in polyclads was studied by means of transmission electron microscopy and histochemistry. Shell-forming granules (SFG) in the egg, as well as secretions of shell glands (SGS), play roles in eggshell formation. As the oocytes pass through the portion of the female tract where the shell glands open, they are surronded by a two-layered envelope of SGS . This envelope prevents the dispersion of SFGs discharged after oviposition, and its inner layer participates in eggshell formation with the SFGs . In Pseudostylochus sp ., most SFGs consist of five parts . Similarities in staining between the parts of the SFGs and the parts of the eggshell indicate that discrete parts of the shell are derived from specific SFG components . Hardening of the eggshell and egg-plate matrix takes place through primary tanning of a sclerotin-like protein .
The spermatozoa in the testis of three species of triclad turbellarians showed slightly different morphologies from spermatozoa in the seminiferous tubule and ovovitelline duct, and we suggest these differences relate to maturation of the spermatozoa. Glycogen granules could be found in the flagella between the central core and the peripheral doublets of spermatozoa in the seminiferous tubule and ovovitelline duct but only rarely in spermatozoa in the testis, and connections between a third arm of the doublets and the flagellar membrane were detectable only in those spermatozoa in the ovovitelline duct and seminiferous tubule. The movement of spermatozoa also appeared to change with capacitation: spermatozoa isolated from the spermiducal vesicle swam with either the anterior process or the head leading; those within the ovovitelline duct appeared to move with the anterior process leading. The flagellum, with the so-called '9 + 1' axonemal pattern, was observed to divide into nine branches toward its distal end; the central cylinder of the axoneme did not continue into the branches. An acrosome has not been observed in planarian spermatozoa. Rod-like structures and a dense mass lying in the anterior process and suspected of containing an acrosomal substance, could not be shown to produce acrosomelike proteolytic activity.
The visual systems of Bdellocephala brunnea Ijima & Kaburaki, a species with two eyes, and Polycelis sapporo (Ijima & Kaburaki), a species with multiple eyes, were investigated using light and electron microscopy. The eye of the binocular species consisted of 40-50 photoreceptor cells and 6-12 pigmented eyecup cells. The eye of the multi-ocular species was smaller and consisted in most specimens of one photoreceptor cell and one pigmented eyecup cell. The ultrastructure of the photoreceptor cells and of the pigmented cells was similar in the two species. Despite differences in numbers of constitutive cells, the arrangement of functional elements in the ocelli of these planarians is the same.
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