hypotheca


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hypotheca

[¦hī·pō′thē·kə]
(invertebrate zoology)
The lower valve of a diatom frustule.
Covering on the hypocone in dinoflagellates.
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A specimen 640 [micron]m long and 20 [micron]m wide, with a gradually tapering hypotheca ending with a thin theca with an acute end without spines, presumably belonging to A.
Cells long and thin, fusiform, with a thin "neck" and a straight hypotheca, in lateral view inflated in its middle part (midbody), gradually diminishing backward and only slightly widening at the truncate posterior end and bearing four spinules, directed posteriorly and almost equally distanced.
Freshwater dinoflagellate, small, ovoid topentagonalcell, plates thin, plate pattern: apical pore, pote plate, canal plate, 4', 2a, 7", C6, S5, 5"', 2"", most photosynthetic with yellow-gold plastids, cingulum is wide, sub-median and the hypotheca is smaller than the epitheca.
TABLE 1 Differences between Peridinium cinctum and Parvodinium umbonatum Feature Peridinium Parvodinium cinctum umbonatum # apical intercalary plates 3 2 # cingular plates 5 6 Size-length 40-64[micro]m 16-28[micro]m Size-width 33-58[micro]m 12-26[micro]m Apical pore no yes Hypothecal spines no sometimes Sulcusin hypotheca parallel sides widely spreading
Cells are large, cuneate, with low, broadly rounded epitheca, much narrower rounded posterior portion of the hypotheca, and nearly straight or slightly convex (sometimes slightly concave) sides of the hypotheca, with coarsely reticulate theca.
Hypotheca is mostly with convex sides, with well-separated antapical bulb directed posteriorly.
Different morphology of the sulcal list can be related to the phenotypic variability, whereas the variation in shape of the hypotheca is expected to be more conservative.
Features such as cell form, the number and arrangement of thecal plates and the presence of spines on the hypotheca of these specimens did match those of Peridinium quinquecorne; however, the number of spines did not.
The abbreviations for cell measurements are as follows: L--total length measured from the furthest part of the hypotheca including the antapical horns; Wb--cell body width measured in ventral or dorsal view just in front of or behind the cingulum, not considering the precingular or postcingular membranes (the only measured width in the species with straight or slightly curved antapical horns directed backward; always measured using the 40x objective to avoid a large error); Wt--total width, considering the distance between external sides of the antapical horns directed laterally, forward or laterally-forward.