Some definitions

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Notwithstanding improvements in the current classification to describe living things, everyday language, including that which biologists speak, uses a number of terms that derive from old classifications based on the shape and behavior of organisms (Figure 63 & Figure 64). If they do not fully reflect reality, they are easy to use and allow you to quickly understand what type of organisms you are referring to. Some terms reflect monophyletic groups well, others do not. Here are some definitions for the most used terms.

Animals are most often defined as multicellular organisms exhibiting an extracellular matrix formed of collagen (or exhibiting “a basal lamina”). This definition is that of metazoa (Metazoa). Some biologists prefer to stick to the old definition and add that animals must be more responsive. Others argue that they must additionally feed by ingestion. Animals must therefore have nerves, muscles and a digestive system (mouth and stomach). The sponges (Porifera) and related organisms devoid of reaction, without muscle, neither nerve, nor true digestive cavity, are qualified as parazoans while the rest of the metazoans possessing muscles, nerves and digestive system are said to be eumetazoans. According to some biologists, animals are therefore either metazoa or eumetazoa! While metazoans form a monophyletic group, recent data suggests that eumetazoans may be polyphyletic. The study of animals is the prerogative of zoologists.

Plants photosynthesize and are made up of a corm, which is a multicellular structure with a stem and leaves, and many of them have roots. Plants produce multicellular embryos which are protected in multicellular tissues, hence their name embryophytes. It is a monophyletic group. Their study is done by botanists.

Protists were first defined by Haeckel as organisms which are neither animals nor plants. They therefore grouped together a large polyphyletic assembly of prokaryotes and eukaryotes. Currently, researchers restrict them to eukaryotes, possibly under the alternative terminology of protoctist. This is the definition that is used in this web resource. Protists are either of the protozoan, algal, or fungal type.

Protozoa are all organisms that spend most of their life in unicellular or colonial form and possess phagotrophic-type nutrition. Some, despite their name, can sometimes make multicellular structures, especially at the time of reproduction. Their study is made by protozoologists. Classically, the following types are distinguished:

• amoeba or amoeboid, also known as rhizopod or actinopod, which characteristically moves using pseudopodia. This cell type consists of many variants with single or multiple pseudopods, lobed or threadlike or even reticulated, etc., including the plasmodium which is a giant amoeba with several nuclei (up to several billion!) and the radiolar and heliozoan types which are radiating amoebae with pseudopods specialized in capturing prey. Certain species are protected by tests or thecae.
• flagellate, that moves using one or several flagella. The number and position of the flagella make it possible to differentiate different subtypes: unikont cells with one flagellum versus bikonts with two flagella, or polykonts, opisthochonts with the flagella at the back of the cell versus anterochonts with the flagella at the front of the cell. Heterochonts have flagella having distinct shapes, unlike isochonts.
• ciliate, which moves with the help of cilia that cover the surface of the cell. The cilia have the same structure as the flagella, but their axoneme is shorter and they are present in very large numbers. Amoeba, flagellates and ciliates are cell forms and do not represent monophyletic groups. Many organisms can adopt or differentiate several of these forms during their cycle: for example the “amoeboflagellates” that can adopt the amoeba form or the flagellated form depending on the environmental conditions.

Algae are organisms that are able to photosynthesize but are not plants. It is therefore a polyphyletic assembly which brings together very diverse eukaryotic organisms resulting from primary, secondary, etc. endosymbioses of the plastid. Previously, they also included prokaryotes such as “cyanophyceae” or “blue-green algae” currently known as cyanobacteria. These are now excluded, so algae are only eukaryotic. Unicellular microalgae are distinguished from multicellular macroalgae. There are flagellated algae and others that are not. Some can still feed in part by phagotrophy; these are mixotrophic algae. None of these categories are monophyletic! Note that the study of algae is done by phycologists.

Fungi eat an osmotrophic type of diet and often feed on detritus that they digest outside the cell. It is a polyphyletic assembly which brings together “true” (Eumycota) and “false” (Pseudomycota) fungi. Note that true fungi are close relatives to animals and further away from plants, and the opposite is true for false fungi! We mainly distinguish between yeasts, which are fungi in unicellular form, and molds which produce a characteristic colonial or multicellular thallus, the mycelium. This thallus is made up of hyphae which can be with (septate thallus), or without partitions (siphoned thallus). Yeast and molds can constitute different stages of the life cycle of the same species. They do not represent monophyletic sets. The study of fungi is the prerogative of mycologists.

While the terms defined above can be used to quickly describe organisms, they do not reflect the kinship relationships between different species because most do not cover a monophyletic set. These have generally only been defined very recently using data from the sequencing of genes, and even genomes. These results call for the adoption of a nomenclature that will ignore the above terms, although they are very practical for describing organisms.

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