Feeding apparatus

          In Euglenoids, the feeding apparatus can be grouped into one of four types, the MTR/packet type (Type I) as found in Petalomonas cantuscygni and as a vestigial structure in Euglena gracilis, the plicate type (Type II) as in Ploeotia costata, the short extensible type (Type III) as found in Perenema trichoporum, and the last, the siphon type (Type IV) in Entosiphon sulcatum.

       The Type I feeding apparatus has the least structural complexity. It consists of a cytoplasmic pocket which extends posteriorly for roughly one half to three quarters the length of the cell. At its anterior end, the feeding apparatus merges with the flagellar pocket. The feeding pocket is lined by microtubules which extend along its entire length, and appear to be derived in part from one of the microtubular flagellar rootlets. The Type I feeding apparatus is restricted to bacteriotrophic euglenoids, the euglenozoan Diplonema and as a vestigial form, or MTR/pocket in many photosynthetic euglenoids such as Euglena gracilis.
 

        The Type II feeding apparatus consists of a cytosome that is supported by two rods which extend into the cytoplasm and a series of plicate folds, the vanes. At the anterior end of the apparatus the vanes surround the membrane invagination which forms the cytosome. For most of the length of the feeding apparatus a portion of the vane complex remains closely appressed to the supporting rods. At the base of the feeding apparatus the rods and vanes are intimately associated with one another forming a single complex. As in the case of Type I, the Type II feeding apparatus is typically associated with bacteriotrophic taxa such asPloeotia.

        The Type III feeding apparatus consists of a cytosome surrounded by vanes and supported by two rods which are composed primarily of microtubules. Unlike the Type II feeding apparatus, in Type III the vanes do not remain appressed to the rods. The number and arrangement of microtubules in the rods varies between genera. Additionally, several fibrillar components can be associated with a Type III feeding apparatus. An example is the distinct striated fibers that extend from the rods toward the vanes near the anterior end of the apparatus in Dinema. Organisms with a Type III feeding apparatus are capable of engulfing eukaryotic prey.

        The Type IV feeding apparatus is found only in the bacteriotroph Entosiphon sulcatum.  It is similar to apparatus Type III, having supporting rods composed of closely packed microtubules and four centrally located vanes surrounding the cytosome.  However, near the base of the feeding apparatus, one of the rods splits giving rise to a total of three rods which extend nearly the length of the cell. About one-third the distance down the apparatus (from the anterior end) the number of microtubules per rod increases dramatically and then decreases in number toward the base giving the apparatus the overall appearance of a cone with one side open. Also, unlike Type III apparatus, Type IV feeding apparatus is usually in motion, extending toward the anterior of the cell and then withdrawing down into the cell for a distance of 3-5 µm.

    In addition to these four types of feeding apparatuses the MTR/pocket complex is considered to be a type of feeding apparatus. Consisting of a pocket formed from the reservoir membrane, a band of microtubules (MTR) that line the pocket membrane, and a dense fibrillar mesh associated with the membrane, its structural similarity with the Type 1 feeding apparatus of colorless euglenoids and bodonids suggests that this pocket is a cytosomal homolog that may or may not serve some function in photosynthetic taxa.