Each of the six essentially different form units, which we, in the foregoing, have distinguished as six different orders of morphological individuality, appears in certain organic (biological) species as physiological individual or biont. In a section of our introduction to Tectology we had indicated with this expression that uniform spatial magnitude, which, as living organism, as more or less centralized life unit, is able to independently exist during a longer or shorter period of time. An existence, expressing itself in all cases by the performance of the most general organic function, namely of self-maintenance by means of metabolism. Also other life functions can be performed by the physiological individual : Reproduction or maintenance of the species, as well as to maintain its relations with the environment, for example by locomotion. But the actual performance of these functions needs not necessarily enter the concept (definition) of biont (i.e. in order to be a biont or physiological individual it does not need to constanly execute those functions. Only the function of self-maintenance must constantly be executed). The biont or functional individual is accordingly not (in contradistinction to the morphological individual) an indivisible spatial magnitude, which we, at the moment of consideration (assessment), should see as unchangeable (indivisible in the sense that we cannot take away a part of it without destroying its character as form individual). Rather the physiological individual is a uniform coherent spatial magnitude, that we see living during a longer or shorter period, i.e. that we see maintaining itself by metabolism, and which we accordingly view at the moment of consideration as changeable. Also parts of the functional individual can separate themselves from it, without its individuality, i.e. its continued existence as independent life unit, becoming to be at risk, and when the biont reproduces, this separation of parts indeed takes place regularly, parts that can develop into new bionts. Accordingly we can summarize the important difference between morphological and physiological individuality as follows :

The  p h y s i o l o g i c a l  i n d i v i d u u m  (biont) is a single organic spatial magnitude, which, as centralized life unit is capable of self-maintenance and at the same time is divisible, and which, in virtue of the actions that are associated with these functions, can only be considered (assessed) as changeable at different points in time.
The  m o r p h o l o g i c a l  i n d i v i d u u m  (first- to sixth-order) on the other hand, is a single organic spatial magnitude, which as a completely delimited form unit is indivisible, and which as such can only be seen, at a particular moment in time, as something unchangeable.

Each of the six morphological individualities of different order, which we have characterised in the previous documents, is able to represent the physiological individuality, and each biont, which as a mature representative of the species, possesses a higher degree of morphological individuality, must have passed, if it develops from a fertilized egg or from a non-fertilized cell (spore), through all the previous lower degrees of morphological individuality during its developmental cycle.
Our task here is to show that each of the six degrees of morphological individuality can function as biont. But before we set out on that task and scrutinize all six orders of individuality, it is necessary to distinguish between three essentially different manifestations or sorts of physiological individuality, which generally can be signified as the actual biont (or biont in the narrow sense), the virtual or potential biont, and the partial or apparent biont.

An  a c t u a l  b i o n t  or physiological individual in the narrow sense can be called every completely developed organic individual, which has reached the highest degree of morphological individuality, that is entitled to it as mature completely developed representative of the species.

This degree is a definite one for every organic species. For example the actual biont in higher plants is a six-order morphological individual, in Vertebrates it is a fifth-order, in most Molluscs it is a fourth-order, in Volvox, and in many Foraminifers it is a second-order, in unicellular organisms, finally, it is a first-order morphological individual. Only with respect to the third-order form individual (antimers) it is not certain whether it can, and if so, whether it indeed does, occur as physiological individual.
There are of course some organismic species that experience in their final developmental stage (the stage in which they can reproduce) a partial reversal to a lower degree of morphological individuality. But these are exceptions caused by nature's exploration of the space of possibilities of development. Almost every such possibility is tried out by her in order to be able to occupy and utilize every niche present on this remarkable planet. Nature can and will deviate from "types" when necessary and experiments into all directions.

A  v i r t u a l  b i o n t  or potential physiological individual, can be called every undeveloped organic individual, as long as it has not yet reached the highest morphological degree of individuality, that the mature individual is entitled to have and to which it can develop.

This degree is different at different points in time and in different stadia or periods of the individual development.
In many organisms every single part of the body can, under certain conditions, behave as, and thus be, a virtual biont, and thus develop into an actual bion, as is the case in Hydra (a fresh water polyp) and in many plants, where many cell groups of the body have such an extraordinary ability to reproduce, that they themselves can, when separated from the parent organism, i.e. from the actual biont, again develop into such a whole.

A  p a r t i a l  b i o n t  or apparent physiological individual can be called any part of an organic individual, that possesses, after its separation from the potential or actual biont, the ability to maintain itself for a longer or shorter period, and to continue its existence as apparent independent biont, without, however, to be able to develop into the actual biont.

The apparent or partial biont can never (as virtual bionts indeed can) develop into the whole organism, i.e. gradually develop into the actual biont by independent growth. Rather it will perish after it had maintained itself for some time, and sometimes had performed a certain function (for instance reproduction) in this period of time. Such is the case for example with the hectocotylus (an organ) of the Cephalopods, the proglottis (a metamer) of Cestods (tapeworms), which have separated themselves from an actual bion of higher order (Haeckel, 1866). As one sees, this concept of partial or apparent biont is a very broad and undetermined one, and it is by far not as significant as are the essential different virtual and actual bionts. However in earlier (i.e. before 1866) attempts to determine the organic individuality, the partial biont was considered to be very important, so it is perhaps not superfluous to introduce the partial biont as a third manifestation of physiological individuality alongside the virtual and actual biont.
Above we articulated the important statement that each of the six morphological individualities can appear as biont or physiological individual. We must now add that this statement is valid for all three manifestations of the biont. The actual, the virtual, as well as the partial can be represented by each of the six morphological individualities. In this way we get eighteen possible organic life units, with which we will now sequentially deal, starting with the lowest and finishing with the highest order. In each order of morphological individuality, we will first deal with the actual biont, which is the most important, next the virtual biont, and finally the partial biont. We start with the first-order morphological individuum, the  c e l l.

The next Figure depicts yet another second-order physiological individual, a colonial Flagellate.

Figure 8.   Volvox globator, a colonial flagellate.   Main image :   one-half of the spherical colony.   B :   zygote, with male and female nuclei uniting, and protective shell secreted after fertilization.   C :   microgamete or spermatozoön.   Sp. 1, 2, and 3 :  developing sperm-spheres.
( After CURTIS, W. & GUTHRIE, M., 1947, General Zoölogy )

In our consideration concerning organs it should again be stressed that the term ORGAN does have two meanings :   The usual meaning (and also its literal one) is that of a working unit, i.e. a unit that is geared to perform a certain function, like the bird's wing, its heart, its lungs, etc. Another meaning is that of a form unit, i.e. one of the building blocks of the organism's body. In the latter sense it is a second-order morphological individual. Our concept of an organ as PHYSIOLOGICAL INDIVIDUAL (biont) also means a second-order morphological individual, but now one that is not actually subordinated to any higher morphological individuality, but is a subsisting entity, which means that it can maintain itself by its own actions. So here the unity not only refers to a coherent spatial entity, but also to a coherent energetic entity, an work entity, that should be considered during a certain time span in which it often is able to let parts of it split off without losing thereby its condition of a coherent energetic unity.
Let us work out all this, in order to better understand the meanings of the concept of ORGAN.
In every organism, which belongs to a higher-order morphological order, it is easy to recognize as such the subordinated individualities, the metamers, antimers and cells. As ORGANS we will accordingly denote all those uniform form appearances that do not belong to one of the other five orders of individuality. In the tectologically so distinctly determined and differentiated higher plants (phanerogams), vertebrates, articulates, etc. we will not entertain doubts as to which parts of them are organs and which not. However this distinction becomes more difficult in many lower forms of both kingdoms, especially in cryptogamic plants. Especially in these cases it is paramount to distinguish between the morphological and physiological concepts of organ. Only the first gives us the second-order form individual.

The  o r g a n  a s  p h y s i o l o g i c a l  u n i t y  can be an integrating constituent of form individuals of all six orders, and means nothing than a body part which performs a certain function (or is able to do so). So is, for example, a cilium (i.e. a hair that can vibrate) of a ciliated cell (i.e. a cell provided with such hairs, cilia) an organ of that particular cell, and also the stinging thread and capsule of a nettle cell. In every complex organ the subordinated parts, which are cell complexes, are at the same time 'organs of the organ' (On this phenomenon our distinction of the five orders of organs was based). Every antimer is composed of several organs, but can itself be an organ of a metamer, for example the arms of the starfishes. In the same way one can, from a physiological viewpoint, interpret the metamers as 'organs of the person' (fifth-order form individual), and the persons (off-shoots) as 'organs of the colony' (cormus), as we see in articulate worms and in most higher plants respectively. In all these cases the idea of the physiological performance is central when we use the word ORGAN, the idea that expounds the organ as an integrating constituent of a body.

Matters are wholly different with respect to the  m o r p h o l o g i c a l  c o n c e p t  o f  t h e  o r g a n.  This concept invariably means only such an indivisible form unit of constant size and composition, comprising a multitude of cells, which neither can be assessed as an antimer nor as a metamer, neither as person nor as cormus. Such form units indeed not only commonly occur as constituents of all morphological individuals of third to sixth order, but also occur, as physiological individuals, wholly independently, more seldom to be sure as actual bionts, but very commonly as virtual, and sometimes in very conspicuous forms as partial boints. Of the five different orders of organs, that we have distinguished in the section on second-order morphological individuality, the first- and second-order organs (cell fusions and simple organs respectively) occur most often as bionts, more seldom those of third order (composed organs). The fourth- and fifth-order organs (organ systems and organ apparatus respectively) on the other hand, can, according to their nature, never obtain the status of physiological individuals.

The above depicted Diatomean colonies belong to the first-order organs, the form and shape of such a colony is solely determined by the constituent cells and their way of attaching to each other.
Second-order organs have their form and shape partly determined by the whole of which they are a part, so in a way they can, as such, not be distinguished when existing on their own, i.e. as bionts. But, in a certain way they can be so distinguished, namely where the cells have not only just aggregated and attached to each other but also have been changed, i.e. more or less deformed, in order to adapt to a particular overall shape of the colony, as we see it in Volvox (Figure 8). So the Volvox colony is an actual biont having the morphological status of that of a second-order organ. The same is the case in all multi-nucleate Foraminiferans.
Third-order organs or composed (heteroplastic) organs, i.e. cell complexes that consist of an aggregate of dissimilar differentiated cells, and which do not have the positive morphological characters of antimers or other higher-order form individuals, we see appear as bionts in lower organisms, for instance in many Thallophytes (algae and the like) which display a wholly irregular shape, and which are composed of dissimilar cells. Also many sponges are third-order organs appearing as bionts.