Cytomorphology and Ultrastructure
  Outside the Cell
    - Type 1: Simple Cell Membrane
    - Type 2: Cell Surface with Additional Extracellular Material
      - Mucilages and Sheaths
      - Scales
      - Frustule
      - Cell Wall
      - Lorica
      - Skeleton
    - Type 3: Cell Surface with Additional Intracellular Material in Vesicles
    - Type 4: Cell Surface with Additional Extracellular and Intracellular Material
      - First Level
      - Second Level
      - Third Level

These enveloping structures are present in some members of the class Chrysophyceae (Heterokontophyta) such as Dinobryon sp. or Chrysococcus sp. and in some genera of the Chlorophyceae, such as Phacotus, Pteromonas, and Dysmorphococcus. These loricas are vase-shaped structures with a more or less wide apical opening, where the flagella emerge. These structures can be colorless, or dark and opaque due to manganese and iron compound impregnation. We can expect different shapes corresponding to different species. In Dinobryon sp., the lorica is an interwoven system of fine cellulose or chitin fibrils (Figure 2.16). In Chrysococcus sp., it can consist of imbricate scales. In Phacotus, the lorica is calcified, ornamented, and is composed of two cup-shaped parts that separate at reproduction. In Pteromonas, the lorica extend into a projecting wing around the cell and is composed of two shell-like portions joined at the wings (Figure 2.17).

Lorica of Pteromonas protracta.
FIGURE 2.17 Lorica of Pteromonas protracta.

Tree-like arrangement of Dinobryon sp. cells showing their loricas
FIGURE 2.16 Tree-like arrangement of Dinobryon sp. cells showing their loricas.

A siliceous skeleton is present in a small group of marine organisms called silicoflagellates, belonging to the division of Heterokonthophyta. This skeleton is placed outside the plasma membrane; it is a three-dimensional structure resembling a flat basket, which consists of a system of branched tubular elements bearing spinose endings. The protoplast is contained inside the basket and has a spongy or frothy appearance, with a central dense region containing the nucleus and the perinuclear dictyosomes and numerous cytoplasmic pseudopodia extending outward, containing the plastids. Sometimes a delicate cell covering of mucilage can be detected.