Blog Posts: Hydriod
Cnidaria is a large group consisting of approximately 10,000 species, which has been further subdivided into four major classes: Scyphozoa (jellyfish), Anthozoa (anemones, corals, and sea pens), Hydrozoa (hydroids and siphonophores), and Cubozoa (box jellies). Across these many groupings, there exists a variety of life history strategies. All Cnidarians share at least one of two life stages: the polyp and the medusa. Anemones exemplify a typical polyp, and jellies a typical medusa, but these body forms have been coopted across the phylum. Often, species exhibit both forms throughout the life cycle. Additionally, colonial species such as hydrozoans have adopted the strategy of multiple, small polyps known as zooids, which share resources within a colony.
All members of this phylum are unified by the presence of a unique cell type known as the cnidocyte (also known as a nematocyst). The root etymology of the word Cnideria, which is Latin for nettle, is indicative of the function of cnidocytes as a stinging cell. The purpose of this cell is variable, but is most commonly associated with feeding or defense. Because cnidarians generally lack complex motile systems, the nematocyst help to immobilize prey and to keep it near to the body. The general structure of the cell is best visualized as a harpoon: Each cnidocyte contains an organelle called a cnidocyst, which is comprised of a bulb-shaped capsule containing a coiled hollow tubule attached to it. The external side of the cell has a hair-like trigger called a cnidocil, which functions as a mechano- and chemo-receptor. When the trigger is activated, the tubule shaft of the cnidocyst is ejected. This discharge is thought to be the fastest biologically poduced speed, with recent research suggesting the process to occur as fast as 700 nanoseconds, thus reaching an acceleration of up to 5,410,000 g (Nüchter et al. 2006).  After penetration, the toxic content of the nematocyst is injected into the target organism, allowing the cnidarian to devour it.
This group has historically been difficult to study using SEM technology due to the high water content of their tissues. Jellyfish, for example, contain 95% water. Samples require a large amount of preemptive preparatory work prior to analysis, whereby the water is replaced by a fixative and dried using a critical-point dryer or chemical agent. As such, most work has focused on the few hard components left to study, including the nematocyst (Mariscal et al. 1976, ÖSTMAN 1979, Watson & Mariscal 1984, Nüchter et al. 2006) and gravity-orienting cells called statoliths (Knowles 2012). From this work, schemes of classification for cnidarians based on cnidocyte morphology have been developed.  Additionally, research has been directed towards the most economically and ecologically important species, such as corals, which face mass extinctions in the face of anthropogenic change (Mumby et al. 2007, Hoegh-Guldberg et al. 2007). SEM was used to study the cellular mechanics by which coral hosts expel the symbiotic zooxanthellae under thermal stress (Gates et al. 1992).