Calcium oxalate crystals occur in the marine green algae Penicillus, Rhipocephalus, and Udotea, known as producers of sedimentary aragonite needles. In contrast to the externally deposited aragonite crystals which are generally < 15 micrometers long, the oxalate crystals are larger (up to 150 micrometers) and are located in the vacuolar system of the plant. No calcium oxalate was found in the related but noncalcifying genera Avrainvillea and Cladocephalus.

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The thallus of Penicillus is composed of two filament types: axial filaments of indeterminate growth and laterals of determinate growth. In vegetative reproduction new plants arise from horizontal rhizoids. Four stages can be distinguished in development. In the primordium stage the tip of a rhizoid swells and forms a primordium. In the germling stage ascending and descending axial filaments arise from the primordium, the former grow into a fascicle and give rise to lateral saccate branches, each of which forms an ascending and a descending arm and branches further into rhizoid‐like branchlets. Together these structures constitute the foundation of the stipe. At the same time the descending axial filaments elongate and become main rhizoidal filaments with lateral rhizoidal branchlets. In the early juvenile stage the stipe is formed. The elongating ascending axial filaments form a medulla while their laterals produce a cortex. In the late juvenile stage the axial filaments form the capitulum. The Espera state of Penicillus lacks a stipe since the ascending axial filaments do not join in a fascicle. Espera has been grown in laboratory cultures from Penicillus plants collected in the Caribbean region and also been found uncommonly in nature in this area. This state may be a response to environmental stress. The flattening of thalli and their orientation perpendicular to the direction of waves are discussed. A comparison of Penicillus and Codium indicates that at least two types of development exist in multiaxial Eusiphoniidae.

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