The Periodic System Of ARTHROPODA.

Table

The purpose of my report to adduce some new data which confirm the regular character of biological evolution. The data concern the periodic system of Articulata.

The building of the natural system of organisms is one of the principal problems of theoretic biology. According to A.A. Lubischev the natural system is one that reflects the nature of relations between its elements and doesn’t depend on ideological purposes of its author. In such the system all the characters and properties of the object are determined by its position therefore the system is prognostic. For example, D.I. Mendeleev’s “Periodic system of chemical elements” fits the definition.

At their time many biologists inspired by Mendeleev’s success have searched for the similar system. However any attempt was ineffective. It is explained by lacking among biological objects of such category as chemical element. Biologists use to deal with species or their characters. The specific diversity in organisms (and in characters) is so extensive that it may be compared with the diversity of chemical combinations but not with the elements.

Naturally, it is impossible to use species and/or characters as the elements of the periodical system even for a small taxonomic group. That’s why till present days the hierarchical system of organisms is being considered the natural one. Graphically it represents genealogical tree or looking alike cladogram. The hierarchical system completely answers the accepted theory of evolution based on the divergent principle of species origin. However the parallelism phenomenon doesn’t conform to this principle. Parallelism, or recurrence of characters in the species belonging to different taxa, is known for a long time. The resemblance of features and characters in the velvety peach and the sleek one was described by Darwin and is considered now a classical example of parallelism. However only N.I. Vavilov’s works have drawn attention to the serious study of parallelism phenomenon.

While studying the parallelism in the Pamirs cereals N.I. Vavilov discovered the law of homological rows of variability. The law postulates the existence of the definite regularity the form–creating and species features. The presence of row of several characters in species belonging to some genus (family) permits to predict the similar row of characters in species of other genus (family).

The summarizing of homological rows to a table leads to combinative lattice which some authors equated to periodic system. However such resemblance is superficial, as the elements of the combinative lattice are equal and independent, whereas the Mendeleev’s system is correlative.

The investigation of convergent resemblance in the species living in similar ecological conditions concentrated the attention to parallelisms. This trend leaded to the development of doctrine of vital forms (ecomorphs). There are several systems of vital forms for plants and animals. However these systems reflect mainly one diversity aspect – the ecological one. The graphical forms of these systems are combinative lattices or hierarchical trees. In latter cause parallelisms are found while comparing the systems built for different taxa.

S.V. Mejen (1974, 1978) and J.V. Chaikovsky (1990) in their works further developed the field concerned with parallelisms study. Mejen contributed the method of meronotaxonomical analysis to the theory of classification. Like systematics classify organisms the comparative anatomy do the same with organism’s parts. Hence Mejen introduced the conception of a meron, or a class of parts, in analysis. The meron unites the parts (characters) of organism similar in structure, function or properties. The aggregate complex of merons forms an archetype of organism – its general image.

The degree of meron’s expression, in limb for example, may seriously differ in several taxa. The limbs of vertebrates may absent or be rudimentary, may be well– or poor–developed, they may be of different shape and carry out different functions. These meron’s conditions are observed in all vertebrates classes and make up the corresponding parallel rows of variability. More of that in every row the same tendency to meron transformation is exhibited. The conditions of meron forming the row of variability C.V. Mejen called a refren.

While the Vavilov’s law fixed the attention to the recurrence of characters of species belonging to different taxa, Mejen concentrated on the recurrence in the transformation rules of modifying characters in refrens.

J.B. Chaikovsky believes that the refrens regulate the diversity and serve as a tool for its cognition. The study of parallel rows is effective only while comparing them.

Groupping in tables parallel rows formed by some refren one can observe the main tendencies and regularities in modification of characters. Thus new trend in the investigation of diversity phenomenon opens. Chaikovsky calls this trend a diatropics. The prognostic value of the diatropical tables is higher than that of comparative lattices. The regularity of some characters modification is observed only in refrens. "Two taxa may run at different floors but by similar corridors", – notice Chaikovsky.

If the refren demonstrates the consequent order of some character modifications (that not always occurs) the columns position in the table is fixed, while the lines position may left arbitrary because the refrens are independent and equal as well as separate characters in homological rows of variability. The position of the line doesn’t obey the rules of conformation and doesn’t correlates with other characters. In other words, continuing the Chaikovsky’s comparison of the parallel rows with the flours of the building one can say the stair–wells in this building are absent and the floors being equivalent. Thus the same principle of combativeness prevails in the base of diatropic tables.

When in hierarchic systems parallelisms disappear owing to the unequivalence of characters, in the combinative ones hierarchy and correlations dissolve. The diversity of biological objects assumes the existence of all three aspects. Therefore the conceivable natural system must be integral. In addition, according to V.N. Beklemishev, to determine the natural place of organism in general diversity system it must be studied “...at least from four points of view: constructive–morphological, physiological, ecological, and historic ones” (1964, v.1, p.8). The natural system must reflect the laws that influence the structure of diversity and are not known for the most part. The heuristic value of “The periodic system of chemical elements” consists not only of predicting of new elements on its base, but also in ranging the chemical elements diversity. Its naturality was confirmed by subsequent investigations of atom structure and physical regularities in the transformation of chemical elements. It should be noted that the early systems of chemical elements before Mendeleev were hierarchical like biological ones.

When I started my study of functional morphology of crustaceans’ food procuring apparatuses I just didn’t think of building any system concerned with problems of theoretical biology. The main result of my work – “The periodic system of Arthropoda (click here)” – was formed on its own. I believe that it is the main argument for its naturality.

The elements of my system are the morphofunctional schemes reflecting the constructive features of general plan of construction of articulate animals.

Like the periodic system of chemical elements my system represents a table, each cell is formed by crossing rows and columns and contains not more than one morphofunctional scheme.

Just a superficial analysis of the periodic system of Articulata which is intelligible even to non–specialist permits to note some regularities in changes of articulates plan of construction while one form passes to another both down and across. The arthropodologist will get more information, but without knowledge about principles and logic of system construction he will be at a loss and meet difficulties while studying it, if take into account the fact that many of its ideas contradict official position. It may results into distrust and active protest. I hope however that after acquaintance with facts and my ideas described in details in the book some misunderstandings will disappear and others will become an object for discussion.

The periodic system is presented graphically, so I will briefly light up the main principles of its building, its explanation, and consequences from its analysis.

How the system was built.

As I have already mentioned it had come accidentally. For comparative analysis of food procuring apparatuses and plans of construction I began to prepare morphofunctional schemes like these represented in the periodical system but more detailed. Each scheme has been drawn on an individual card. Such cards are very useful for comparative analysis of morphological rows as you may freely move them from one place to another. I call it playing patience. Playing a current solitaire I have unexpectedly built periodical system of crustaceans.

At first the table contained morphofunctional schemes for just main groups of crustaceans. They were organized in three rows corresponding to degrees of complication of food procuring apparatuses and nine columns. Morphofuctional schemes of lower crustaceans were located in the last row. On that stage the table allowed to trace the main regularities while transfer from one morphofunctional scheme to another in rows and columns. This made possible the correction of morphofunctional schemes. For example, the strict logic of the system has made me revise the working principles of filtration apparatuses of the majority of crustaceans and allowed revealing essentially new and common for all apparatuses pattern of filtration. In accordance with that pattern morphofunctional schemes of plans of construction for most crustaceans were revised. The logic of changes in complicating food procuring apparatuses and plans of construction became visible in the course of system building. The system developed by itself as my notions on morphogenesis and functions of food procuring apparatuses and general crustacean plan of construction specified. At some moment the system almost totally took the initiative. It happened after using regularities in remakes of crustaceans plans of construction from upper to lower row I had created the fourth, the lowest row of morphofunctional schemes and tried to find the corresponding forms among animals. But none of these schemes corresponded to features of known to me present groups of crustaceans. Among fossil ones as far as I knew these forms didn’t exist either. Automatically I imagined them only as crustaceans.

Having been slightly disappointed in prognostic abilities of the system I started the analysis of the table. This work made me several times return to morphofunctional schemes of, as I thought, hypothetical crustaceans. To that time thanks to comparative analysis of crustaceans’ plans of construction I already could imagine not only the plan of construction but even the appearance of my hypothetical forms. While looking through “Invertebrata fossiles” (Moor et al., 1952) I came across pictures and descriptions of pseudocrustaceans and was highly impressed of how these arthropods coincided my ideas of hypothetical crustaceans realized in morphofunctional schemes. It must be said that plans of construction of these primitive arthropods are so simple that there was no need to correct my morphofunctional schemes. Thus I came to Trilobitomorpha and trilobites and later to annelids. It’s interesting that the same inertness of thinking hadn’t allowed me at first to treat Tracheata – their morphofunctional schemes I built later during further analysis of the system. After consuming of tracheate arthropods the table gained the final shape. There is no sense in this report to trace all the way I have made creating this system.

“As the main sphere of animals activity especially in lower ones is the food obtaining, its methods mostly influence the direction of evolution mainly on its earlier stages” V.N. Beklemishev (1964, v.2, p.62).

This citation of V.N. Beklemishev forms the starting point in interpretation of Articulata periodic system. In Beklemishev’s system Articulata includes two subphylla: Annelida and Arthropoda.

As supposed the Life arosed in the Ocean.

At the beginning of Paleozoic (about 600 million years ago) most phylla of Metazoa already existed. The main sources of food were and still are small unicellular organisms (bacterio– and phytoplankton), multicellular (animals and algae) as well as detritus (organic matter appearing in result of organisms death and decomposition). In accordance with food sources they distinguish two main methods of feeding – filtration and grasping of separate objects. Thus animals divide into filtrators and graspers.

The filtration method of food obtaining as widely accepted is primary to grasping, the secondary filtrators (not considered here) being the exclusion.

In scientific publications and manuals one can find most contradictory statements on constructive morphology and work of filtration apparatuses of crustaceans and trilobithomorphs. But obviously none of existing descriptions reflects the reality. I have managed to prove that only four main types of filtration apparatuses exist. Differing in constructive details they all work in accordance with one principal scheme based on the laws of hydrodynamics.

The first type consists of the most complicated and perfect filtration apparatus those of higher crustaceans from eucarids. I call this type supermaxillar filtration apparatus (SMFA).

The second type – maxillar filtration apparatus (MFA)

occurs in peracarids, anaspids and copepods.

The third one – thoracic filtration apparatus (TFA)

– occurs in lower crustaceans: anostracs, conchostracs, notostracs, cladoceras, cephalocarids and phyllocarids.

The fourth type – gnathobasical food procuring apparatus (GFPA)

found in trilobithomorphs – is not filtration one in its sense but the same principal scheme of filtration as in upper ones is realized there.

Annelids food procuring apparatus (AFPA)

is not a filtration one but food procuring apparatuses of earlier larvae of annelids function in accordance with hydrodynamic patterns typical to all filtration apparatuses.

On the base of morphofunctional analysis I have postulated that food procuring apparatus of annelids larvae is the ancestral for others. The sequence of changes is the following:

AFPA GFPA TFA MFA SMFA (1)

Each of that types of filtration apparatuses is ancestral for corresponding grasping apparatus. Cannon and Manton (1928) were the first to describe the character of morphofunctional reorganization while chenging filtration for grasping in peracarids and anaspids. Later the similar changes were described in most crustaceans groups by other authors. Usually such transitions has been illustrated by morphological rows or sequence of vital forms from filtrator to obligate grasper. As a rule transition from one ecomorpha to another may be linked with successive adaptation of some group to main marine biotopes. This sequence may be represented as:

PF PG BG EG (2)

Were PF – pelagic filtrator; PG – pelagic grasper; BG – bental grasper; EG – epigeous grasper. Besides these some other groups are usually defined: PFG – pelagic filtrator and grasper at the same time; PBG – pelagobental grasper; AG – amphibian grasper and so on. In this report I divide all vital forms of aquatic arthropods in four groups.

We can represent the relations between main types of food procuring apparatuses and main vital forms as an ecological lattice.

Each of main types of filtration apparatuses may has several modifications forming the full or incomplete row of vital forms from pelagic filtrator to epigeous grasper.

As a result the isotopic rows are formed (marked by numbers) and each of isotopic row forms appears to be isotopic in relation to analogous form from other rows. Putting isotopic row in accordance with the main types of food procuring apparatuses we get three–dimensional lattice:

Each vital form figured in the table being in this context an adaptive morphological type may be represented by morphofunctional scheme reflecting the general construction of crustacean body – its plan of construction. Thus we link comparative morphological rows illustrating the features of morphological changes of plan of construction from filtrator to grasper with ecological row reflecting the succession in adaptations of taxon to main biotopes and food resources.

Replacing designations of vital forms for morphofuncional schemes of their representatives we get the left part of periodical system – columns A to F. There the morphofunctional schemes of aquatic articulates are located. Larvae form column F.

The next stage is the analysis of the system itself.

One of the important conclusions from the analysis of articulates periodic system is that the evolution of food procuring apparatuses (FPA) makes the basement for evolution of the whole articulate type. During the process of perfecting of food obtaining methods the regular changes of food procuring apparatuses plans of construction occur (sequence 1). In accordance with these rebuilds the general plan of construction change in articulates and the new level of organization is gained.

Constructive and functional features of articulate plan of construction are in periodical dependence on the main type of food procuring apparatus.

Let’s look at some of these regularities.

In most simple cases (most annelids) the body of articulated animal consists of acron, a series of similar segments bearing a pair of limbs (parapodes) and anal lobe (look forms A – F in periodical system) Mouth and organs of food procuring are located on the first segment. This first segment differs from the others. In combination with acron the first segment of annelids body forms the primitive head part which I call procephalon. Its formula is:

Acr + A

None of annelids limbs take part in food procuring.

Annelids type of organization is ancestral to the first group of arthropods – trilobithomorphs. The most important difference of arthropods from annelids is that limbs of the former are used in food procuring.

The first two segments in all arthropods are joined together with acron forming the primary head (protocephalon). In trilobithomorphs from two to eight segments may be fused. This structure is usually treated as a head.

In the definition of head part I use the character of idiosegmentation instead of widely accepted criterion of fused forward segments. Idiosegment by V.N. Beklemishev (1964) is any segment having individual characters. That’s why instead of term “head” in crustaceans I use “the head functional complex (=head part)”.

The limbs of the first segment of trilobithomorphs body as in most arthropods are differentiated for carrying sensor function. Limbs homologous to the second antennae pair of other arthropods ensure the direct transfer of food objects to the mouth differing in that from other limbs.

Segments bearing the first and the second pairs of antennae are idiosegments according to definition. Limbs of the third segment (homologous to mandibular segment of crustaceans) in the most primitive trilobithomorphs differ from others only by their position relatively to protocephalon. This segment appears to be transitional and is characterized by illegible features, such segment I mark with +. So the formula of trilobithomorphs head functional complex will be:

Acr A₁A₂ + Md

Formulae for head functional complexes of other arthropods can be found similarly.

Comparing head functional complex formulae one can find out that the quantity of segments in them grows while uplifting of level of organization in odd numbers (look the system). The number of subdivisions in head functional complex also increases. (Tab. 3)

Naturally increasing of segments in head part leads to their decreasing in thoracic part. This regularity is not so strict as the former one because the decreasing of segments in thoracic part happens in many arthropods together with total oligomerization.

Idiosegmentation effects in prominent phenomenon of increasing of heteronomous metamerism degree while uplifting of level of organization. The process of heteronomous metamerism development in arthropods was described in details by V.N. Beklemishev (1964) but the causes of that process remained unknown to him. The character of link between food obtaining function and this process is thoroughly shown in my book.

In aquatic arthropods developing with metamorphosis the quantity of basic larval stages is equal to the number of level of organization. The basic larval stage is the series of stages going without changes in food procuring apparatus of larva. There are four of these stages (look the system). There are some other regularities not represented in this reduced version of the system.

While in columns one can trace peculiarities in changes of the general plan of construction, in rows at each level of organization regular changes in structure and function of apparatuses and organs caused by adaptations to main biotopes and food resources take place.

The appearance of isotopic forms on each level of organization is also submitted to certain rules of transformation of structure and function of apparatuses and organs.

In fact, giving coordinates of any cell of the system we can on the base of deduced regularities of transitions in rows and columns reconstruct the unknown form or exhaustively explain its lack.

Just these features of the system intended at first only for crustaceans allowed create the periodical system embraced all the type of articulates.

However, before it I had to solve the problem of transition from one level to another. Comparing analogous forms from neighboring levels of organization, for example forms III–C and IV–C, we come to the conclusion that the second one can’t be drawn from the first. It applies equally to all such pairs. At the same time such transition is easily observed in ontogenesis. The change for new level of organization in larva (or juvenile stage) proceeds through pedomorphosis and following neoteny.

Garstang was the first to elaborate this model for transition from annelids to early arthropods but in this speculations he used only changes in locomotive apparatus. In my reconstruction of this transition both locomotive and food procuring apparatuses are involved. More of that, it appeared that the leading role belongs to food obtaining and mostly filtration function.

Filtration apparatuses and their modifications nearly without exceptions can be transformed to grasping ones. There are no obligate filtrators but obligate graspers are common. This is true for larvae as well.

There are several trends in development of pedomorphic larva (pic. 4). We have considered only one – from filtrator to pelagic grasper, then to bental grasper, amphibian and epigeous. If the pedomorphic larva at once settles it transforms to attached filtrator, barnacles being an example (this trend I haven’t studied).

The most interesting is the third trend – the development of pedomorphic larva in grasperine way with simultaneous settlement on substrate. The reconstruction of these forms leads to tracheates (right part of the system, forms G – J).

While reconstructing of these forms we can use regularities of transition from pelagic to bental grasper according to the first trend. We have to take into account that the transition to the grasper type of feeding will leed to more deep morphological and functional reorganizations of food procuring apparatus as well as the whole general plan of construction of neotenic forms already at larval stage. In that trilobithomorphs and crustaceans differ from other groups of arthropods.

It have to be marked that pedomorphic larva being neotenic have given rise to forms of first insects and myriapods which inhabited substrates of intermediate zone between earth and sea. The role of such substrates could play aquatic plants on shoal and intertidal zone and vegetation matter.

Finally, the fourth trend – the realization of grasping functions of larva in pelagic zone – leads to such mysterious forms as Anomalocaris and others known as fossil trilobithomorphs.

The periodic system gives an opportunity to have a new look on phylogenetic relations between major groups of articulates (pic. 5). The picture shows that all transitions from one level of organization to another are realized in one way.

1. Basic larval stage
2. Pedomorphosis
3. Neoteny
4. Gaining of adaptive zones
JZ - Intermedial zone