Evolution As a Basis for the Systematization of Nematodes - a Critical Review and Expose
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EVOLUTION AS A BASIS FOR THE SYSTEMATIZATION OF NEMATODES - A CRITICAL REVIEW AND EXPOSE BY A. COOMANS Rijksuniversiteit Gent, Instituut voor Dierkunde, Ledeganckstraat 35, Gent, Belgium In a recent book 1) Andrassy presents a classification of freeliving nematodes based on their phylogeny. Since few fossil nematodes are known this phylogeny can at present only be derived from recent forms. By comparing nematodes with related groups their position in the animal kingdom can be defined or at least discussed. Comparative morphology can yield much information when properly as as applied. Therefore it is essential that the characteristics of nematodes well those of the groups compared with them are critically analysed. Listing among the general features of Nematoda that the first moult of a juvenile occurs within the egg (p. 24 and 41), that the cuticle is .rhed five time. (p. 24), that they lack cilia (p. 24, 2 and 31; the discovery of cilium-like structures is only mentioned in a footnote p. 24) and have smooth muscles (p. 25, 31 and 33) are disturbing errors. Indeed a first moult within the egg has only been recorded for Secernentea, nematodes moult only four times and their muscles are obliquely striated. There is now ample evidence that they possess non-motile, sensory ciliary structures. - Kinorhyncha also have such modified cilia and contrary to Andrassy's state- ment - may have a triradial pharynx. Stating that cell division after the first cleavage is specific izz its sequence in the izem,itodes ( p. 2 5 ) is denying the resem- blances in embryology with Gastrotricha. Using the term protocoel instead of pJeudocoel is confusing since it also means the anterior part of the coelom in e.g. Tentaculata. Most of the phylogenetic hypotheses about nematodes formulated so far are basically similar and differ only in details, hence they can be grouped as one with several variations. Andrassy's "theory" is another of these variations. Nematoda are compared with other Protostomia. These comparisons include contradictions and errors. Priapulida are described as having a protocoelom ( = pseudocoel, see above), whereas most specialists now agree their body cavity is a coelom. The 1) Evolution as a Basis for the Systematizationof Nematodes by I. Andrássy - 1976 - London, Pitman Publishing Ltd, 288 pp. price £ 7.90, $ 15.75. 130 fact that Priapulida, Kinorhyncha and Nematoda possess an exoskeleton-like cuticle is one thing, but that the nematode cuticule is proteinaceous and that of both other groups mainly chitinous is another. Ten species of Priapulida are known (instead of four, p. 37) and none of the newly described features support An- drissy's conclusion that there is a definite relationship with Nematoda. There are many contradictions and mistakes. The praesoma (or proboscis, not rostrum!) and pharynx are wrongly described (p. 37) as triradial (they are pentaradial), but on p. 31 a triradial stomodeum is listed as a difference between Priapulida and Nematoda (which is true). Priapulida are said to lack a definite larval form on p. 31, but to possess aciliate larvae p. 39. Acanthocephala some- - times lack an intestine (p. 30 and 39, line 7 true), sometimes have a large - intestine (p. 39, line 31 wrong). Proctodeum is printed (p. 35 ) instead of "stomodeum", lack of ligamental sacs instead of "ligamental sacs", direct develop- ment instead of "indirect development" (both on p. 39 ) . It is not true that there is a razz bisexuality in nematodes (p. 32 where compared with Annelida), that Gastrotricha cannot swim (p. 31), that nematodes are short-lived compared - with Tardigrada (p. 33 life span depending upon species!), that Annelida - lack a respiratory system (p. 32 many aquatic species possess gills!), that Acan- - thocephala have a cylindrical body (p. 39 this is only so in dead animals, living ones having a flattened body), nor that they lack .. , cell constancy (p. 39 ) . Anyone familiar with marine nematodes, observing living nematodes in cultures or aware of the recent investigations on the ultrastructure of sense organs will disagree with Andrassy when he states that nematodes have undeveloped sense organs. Nematodes are described as having the same build-up... by the whole group (p. 24), extrenael y verfatile structural ... characteristics (p. 41 ), cora.rer- vative and unified structure (p. 42 ) . All this can be true depending on the struc- tural level being considered, but in the text it seems to refer to the same level. Twenty different invertebrate groups are compared with Nematoda, their similarities and differences listed. Nematomorpha (Gordiacea) appear the most similar to, Mollusca the most different from Nematoda. On the basis of this com- parison Andrassy tries to construct gradations of relationship. First, it should be noted that only a limited number of characters are considered. More important is that little, if any attention has been paid to the origin and importance of these characters, i.e., whether they are analogous or homologous and hence whether such comparisons are meaningful. It is obvious that convergence and true relation- ship have not been sufficiently differentiated. How, otherwise, can it be claimed that Onychophora and Arthropoda appear more closely related to Nematoda than Annelida? Cestoda and Trematoda more than Turbellaria? Priapulida more than Gastrotricha? How could Kinorhyncha originate from Desmoscolecida? Very little if any attention has been paid to the relationship between form and function. The outstanding features of the nematode bodyplan are best explained by functional morphology; also some seemingly important differences with related can groups be better evaluated. The embryology of nematodes is rather typical, .