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Hundreds of active and dormant geothermal vents have been located on the floor of Yellowstone Lake, although characterization of the associated biology (macro or micro) has been extremely limited. Herein, we describe an aquatic moss (Fontinalis) colony closely associated with vent emissions that considerably exceeded known temperature maxima for this plant. Vent waters were supersaturated with CO(2), likely accommodating a CO(2) compensation point that would be expected to be quite elevated under these conditions. The moss was colonized by metazoa, including the crustaceans Hyalella and Gammarus, a segmented worm in the Lumbriculidae family, and a flatworm specimen tentatively identified as Polycelis. The presence of these invertebrates suggest a highly localized food chain that derives from the presence of geothermal inputs and thus is analogous to the deep marine vents that support significant biodiversity.

The heterocellular female gonad of the typhloplanoid Castrada viridis consists of a single germarium and two rows of vitellaria. The germarium, composed of a germinative zone and a growth zone, is surrounded externally by a layer of accessory cells the function of which is hypothesized in this study. The main feature of oocyte differentiation is the synthesis of small electron-dense inclusions produced by the rough endoplasmic reticulum (R.E.R.) and Golgi complex. The electron-dense content of the egg inclusions reacts positively to the cytochemical test used to detect polyphenols and is only partially extracted following incubation in protease. The genesis, composition, and peripheral location of egg inclusions in mature oocytes suggest that they could represent residual eggshell granules. The presumed function of eggshell granules is discussed and their fine morphology is compared with that observed in other neoophoran Platyhelminthes.