genus to he, indeed, the connecting link between annelids 

 and nematodes and for many years the Desmoscolecidae 

 were excluded from the Nematoda or considered an aber- 

 rant appendage. Not until Schepotieff (1908) studied the 

 group and found its members to be internally of typical 

 nenuc structure, were the Desmosculecidae accepted as 

 part of the Nematoda. 



3. Arthropod Theories. In reality there seem to be 

 I.WO schools of thought regaiding namic-arthropod re- 

 lationships; the first school provides for the nematodes 

 through degeneracy, the other throug-h ascendancy, but 

 at times the two theories seem to merge! 



A. Degeneracy. Perrier (1897) seems to have ori- 

 ginated the idea that free-living nematodes should be 

 legarded as having originated secondarily from para- 

 sitic forms. The viewpoint was at that time more or 

 less natural since the majority of zoologists were ig- 

 norant of free-living nematodes. Perrier's series Chi- 

 tinophores, based on a "chitinous covering" and molts, 

 included the Nemathelminthes with classes Nematoda, 

 Acanthocephala and Gordiacea. This group was set 

 entirely apart from the series Nephrides which included 

 Rotatoria, Gastrotri;ha, Turbellaria, Trematoda and 

 Cestoda. 



Hubrecht (1904) followed with a theory in- which it 

 was plainly stated that free-living nematodes are much 

 too "simple" to be archaic stem forms and must, instead, 

 be secondarily degenerate products of the Arthropoda. 



Rauther (1909) thereafter frankly invoked neoteny 

 to explain all things in anatomy and relationship of 

 worms. A direct quotation relating to possible gordiid- 

 echinoderid relationship will serve best to illustrate 

 this course of thought (p. 502). "der Go)-dius-La.rve 

 schreibe ieh ebsowenig- die bedeutung einer Rekapitulation 

 einer primitiven Gordien-Stammform zu, wie ich in 

 den aquivalenten Larven der hohern Wiirmer und Mol- 

 lusken Abbilder von deren Vorfahren sehe. Die Auffassung 

 der Echinoderiden als neotenis.her Articulaten-Abkomm- 

 ling'e wird ferner gerade durch ihre unzweifelhaften Aehn- 

 lichkeiten mit Nematod«n gestiitzt (s.u.) welche letztern, 

 wie sich im Verlauf dieser Ausfiihrungen ergeben diirfte, 

 selbst in analogem Verhaltnis zu Articulaten-Vorfahren 

 stehen." Further on, in the same publication he de- 

 termined that the rotatorian is not a primitive form 

 but the neotenic branch of a higher group reduced to 

 the trochophore stage ; that the nematode developed from 

 a land-dwelling arthropod and that the search for an- 

 cestral groups is useless since they are phantoms. 



Rauther's comparison of the triradiate pharynx of 

 Anopheles larvae with the triradiate nematode esophagus 

 and tardigrade pharynx is ingenious. He placed con- 

 siderable weight on the stylet of tylenchids as compared 

 to the paired stylets of tardigrades and dipterans; 

 molting, rachis development of gonad, and rectal glands 

 (as compared with malpighian tubules) were also cited 

 as evidence of relationship. The Pentastomida ( Lingua- 

 tulida) were placed by Rauther as an intermediate group 

 constituting a connecting link between nematodes and 

 dipteral! larvae. 



Martini (1908) after previously (1903, 1907) estab- 

 lishing that meromyarian nematodes are ontogenetically 

 primitive, switched to the view that the rhabditoid larva 

 is an ontogenetic stage and Rhabditis is neotenic rather 

 than primitive. 



In the face of growing knowledge of free-living nema- 

 todes Keilin (1926) reiterated the "theory of degeneracy" 

 without adding any new facts or thoughts. 



This theory has cast its shadow over all nemic classi- 

 fication since it simplifies grouping so greatly. If, as 

 Rauther suggests, ontogenetic stages are of no signi- 

 ficance, then one may sidestep all difficult points. Con- 

 cerning the invocation of neoteny to explain evolution 

 within the Nematoda, Chitwood (1937) has stated, "Onto- 

 geny supports the view that few-celled forms are more 

 primitive than many celled forms. In the writer's opin- 

 ion, the converse assumption removes the study of nemic 

 phylogeny from the realm of logical thought." 



De Coniuck (19.38) has recently resurrected the term "Eutely" 

 given by Martini (1923) to the phenomenon of cell constancy. 

 He also made clear the distinction betwen this phenomenon and 

 neoteny, attributing eutely to "very rapid segregation of all 

 potentialities of the egg" while neoteny "is the result of hormonal 

 deficiency". As to the distinction, the writers are in complete 

 agreement. 



n. Ascrndavcy. Those who have subscribed to the as- 

 cendancy idea have not necessarily believed that nema- 

 todes gave rise to arthropods but only that nematodes 

 may be a branch coming off from the stock which even- 

 tually gave rise to insects or that free-living nemas 

 developed from an insect-like ancestor and later gave 

 rise to para.sitic nemas. It should be noted that in no 

 instance has an attempt been made by such observers 

 to explain aquatic arthropods. Instead, it is appar- 

 ently assumed that the Arthropoda must be a poly- 

 phyletic group. 



Greeff (1869) being impressed with the secondary 

 and superficial segmentation of marine nemas such as 

 Desmoscolex, Greeffiella, and Draconema believed that 

 these foi-ms together with the Echinodera provided a 

 connecting link between the Nematoda and the Arthropoda. 



Biitschli (1876) supplied a much more substantial ar- 

 gument for common parentage of the two groups. As like 

 characters he cited the absence of ciliation, occurrence 

 of molting, presence of a nerve ring and ventral median 

 nerve; he also homologized the trachea of insects with 

 the lateral canals of nematodes and the nephridia of an- 

 nelids. Biitschli further suggested that the caudal furcae 

 of echinoderids may correspond to the arthropod foot 

 and incidentally placed the Tardigrada as low arthropods. 

 A diagram of the family tree, as he conceived it, is to 

 be found in figure 146. Differences in the mode of 

 jointing in arthropods and annelids, differences in cleav- 

 age and in the vascular systems caused him to separate 

 them into two stem lines. 



Ganin (1877) studied the nervous system of Rhabditis 

 and because of the circum-esophageal commissure, double 

 subventral nerve trunks and ventral chain of ganglia, he 

 placed the Nematoda in the general arthropod series. 



Maupas (1899) emphasized that nematodes molt in their 

 life cycle as do arthropods and he even went so far as 

 to compare the ontogeny of nemas with that of heteroceran 

 lepidopterans in which four molts occur during larval 

 development and two in adulthood. The encysted stage 

 (ensheathed larva, resistant stage, dauer-larva) of nema- 

 todes and its ability to withstand adverse environmental 

 conditions was compared with similar stages in tardi- 

 grades and rotatorians. However, Maupas could not ac- 

 cept rotatorian-nemic relationship because he was unable 

 to find r-otatorians molting. 



Seurat (1920) presented by far the most comprehensive 

 and well-founded arguments for common ancestry of 

 nematodes and arthropods. First he defined the primi- 

 tive nematode on the basis of habitat and comparative 

 anatomy as follows: saprozoic, humid media, bilateral 

 symmetry, mouth subterminal and ventral, three lips, tail 

 thick and conical with three caudal glands, cuticle smooth, 

 sensory papillae sparse, epidermis composed of four bands, 

 four muscle fields, meromyarian, unicellular lateral glands. 

 Mouth tubular, esophagus triradiate, and terminated by 

 a valved bulb, intestine composed of few large cells, oc- 

 casionally a caecum, rectum shor,. with three g-lands. 

 Excretory system with paired lateral canals opening lat- 

 erally or without lateral canals but with a unicellular 

 gland opening ventrally; sometimes with a secondary 

 system opening posteriorly. Sexes separate, males with 

 numerous genital papillae, two testes extending parallel 

 anteriorly and opening posteriorly in a vas deferens and 

 ejaculatory duct; a little anterior to anus large paired 

 cement glands opening into ejaculatory dut; two spicules 

 and a gubernaculum. Female with two parallel ovaries 

 opening posteriorly through simple vagina, oocytes pro- 

 duced in small numbers. Segmentation total, unequal; 

 four molts to adulthood. 



Having stated his concept of the primitive nematode he 

 recognized that such a form would combine characters 

 of oxyurids and rhabditids. A common ancestral stem 

 line with rotatorians and turbellarians would, according 

 to Seurat's view, be eliminated due to the molts, chitinous 

 cuticle, type of musculature, lack of cilia, form of gut, 

 type of reproductive organs, and separation of sexes. All 

 of these characters clearly point toward arthropods. The 

 cuticular lining of the anterior arthropod gut and its 

 differentiation posteriorly into a proventriculus, the 

 existence of a caecum in the mid-gut (=chyliferous diverti- 

 culae), insignificance of hind gut, and the existence of 

 malpighian tubules (as compared with anal glands) must 

 (vide Seurat) be explained otherwise than by con- 

 vergence with nematodes- The male reproductive sys- 

 tem and spicules of nemas are compared with the testes 



191 



