Pseudocoel

In zoomorphology, a pseudocoel is spoken of when the fluid-filled body cavity between the endoderm and ectoderm is not completely surrounded by mesodermal tissue, and if, in addition, no fusion of a secondary with the primary body cavity occurred during embryonic development (the latter would be a mixocoel ).

In contrast to this, the acoelomats do not have a corresponding body cavity, while the eucoelomats are characterized by a cavity completely enclosed by mesoderm, the secondary body cavity or coelom .

Animal groups with pseudocoel are usually grouped together as pseudocoelomats . In the Anglo-American language area, however, the term "blastocoelomatics" is also common, which is related to the embryonic development, i.e. the formation of the pseudocoel from the first cavity of the germ, the primary body cavity or blastocoel (more on this in the section on individual development).

Tube worms (Nemathelminthes, formerly also "Aschelminthes") are an example of organisms that develop a pseudocoel. It is doubtful whether pseudocoelomatics form a natural community of descent; this type of organization probably evolved independently several times. This raises the question of the evolutionary formation conditions of a pseudocoel (more on this in the section on evolutionary formation).

Emergence

Origin during individual development (ontogenesis):

A pseudocoel arises from the primary body cavity (blastocoel), which mixes with the extracellular space between the mesoderm cells. The mesoderm cells are mostly pushed under the ectoderm , where they form layers of muscle. The big difference to the real Coelom is that the body cavity created in this way is not divided and is not surrounded by its own mesodermal cell layer. The organs are exposed in the body fluid.

Different assumptions about evolutionary origin (phylogenesis):

The evolution of a pseudocoel is often explained by the enlargement of the body caused by the accumulation of body fluids in cavities. As an example, tubular worms are cited which, if they are small (microscopic forms), do not have a typical pseudocoel (e.g. Caenorhabditis elegans belonging to the nematode ), but macroscopic forms have a pronounced pseudocoel, the filling of which serves as a hydroskeleton .

This explanation is only half convincing, since the reverse evolutionary course can just as well be assumed, i.e. a secondary loss of the pseudocoel in the course of an evolutionary dwarfing. The pseudocoel would then become unnecessary as a hydroskeletal unit if the body size fell below a certain level and would disappear in the increasingly “tighter” tissue.

In the latter explanatory scheme, the evolutionary origin of the pseudocel in the initial stage would of course remain unexplained. Two remaining hypotheses should be thought through more precisely here: 1) Evolution from an acoelomatic stage, but without any connection with increase in body size as in the first-mentioned explanation, 2) Evolution from a coelomatic preliminary stage, in which the coelom was secondarily abandoned as with the Mixocoel , without however - as with the well-known Mixocoelomats - to leave traces in this regard during the embryonic development or in the adult forms.

Possible terminological consequences from the various assumptions about evolutionary origin

The question to be asked is what terminological consequences result from the various phylogenetic hypotheses. If “pseudocoelomata” have arisen independently several times, a situation could arise, for example, in which all three of the evolution scenarios listed above prove to be correct (i.e. the pseudocoelomas found in each case evolved in very different ways). In this case at the latest, it would be appropriate to be able to use differentiated terminology.

In the traditional morphologically favored view that the pseudocoelomate organization emerged from an acoelomat, nothing speaks against keeping the designation "pseudocoel": This term expresses that the body cavity in question has no connection with real coelomen. However, should the suspicion, which has recently been frequently expressed, be confirmed that the pseudocoelomate organization evolved from a coelomat preliminary stage in some or even all groups, it would be advisable to 1) identify the assumed fusion of the secondary and primary body cavity by using an independent term and 2 ) also to be distinguished from the Mixocoel term, which the latter traditionally denotes a fusion of body cavities in phylogenesis and ontogenesis. The term syncoelom , which is already sometimes used in the literature, would correspond to these two requirements : It conceptually reflects the assumption of an evolutionary fusion of secondary and primary body cavity, even if this fusion - in contrast to Mixocoel - can no longer be observed during individual development.

There is currently no generally accepted terminology that takes into account the various evolutionary interpretations of the pseudocoel.