CHAPTER XIII. 

 NEMIC RELATIONSHIPS 



B. G. CHITWOOD 



Even to enter a discussion of nemic relationships 

 invites criticism and focuses a spotlight on one's ignor- 

 ance of zoology. It is difficult for a scientist to obtain 

 sufficient knowledge to speak with accuracy iconcerning 

 his specialty and whether or not he can do so outside 

 his specialty remains to be seen. Undoubtedly there are 

 as many separate theories as there will be readers of 

 this chapter. Past and present theories, with their evi- 

 dence will be presented. We hope and expect future 

 workers will feel as much superior to us as we do to 

 Bastian when we read "Having now pretty fully ex- 

 plained the anatomy of the Nematoids, we shall be 

 able, with the aid of the many new facts revealed con- 

 cerning their structure, to consider the question of their 

 affinities and homologies with more chance of success 

 than formerly, so that we may hope to throw some 

 light upon this difficult subject." 



Past Theories 



Taxonomic e:ra. The original relationships attributed 

 to zoologic groups were based on casual knowledge of 

 gross anatomy and were typically taxonomic in char- 

 acter. Thus Linnaeus (1758) divided the Animal Kingdom 

 into six classes; Mammalia, Aves, Amphibia, Pisces, In- 

 spects, and Vermes. In the last mentioned group were 

 included all the then known helminths as well as the 

 Protozoa, Mollusca, and Annelida. 



In 1800 Zeder established the groups of parasitic worms 

 essentially as they are recognized today, the groups 

 being Roundworms, Hooked-worms (spiny heads), Suck- 

 ered worms, Flatworms, and Bladderworms; the same 

 groups were named by Rudolphi (1808-'09) Nematoidea, 

 Acanthocephala, Trematoda, Cestoidea, and Cystica, all 

 except the last of which have survived until the present 

 time. 



With the discovery in these animal groups of excretory 

 and nervous systems, and with superficial knowledge of 

 the body cavity and ontogeny, the classifications were 

 expanded by Huxley and Haeckel who sug'gested ap- 

 proximately all of the possible relationships. Huxley's 

 first classification (1856) divided the subkingdom into 

 two phyla, Articulata and Annuloidea, the latter group 

 including the classes Annelida, Echinodermata and Sco- 

 lecida. The entire Entozoa, as well as the Rotifera and 

 Turbellaria, were included in the Scolecida, thus the 

 Entozoa would correspond to the term unsegmentsd worms 

 in the broadest sense. The phylum Annuloidea was then 

 characterized as having a water-vascular system. Shortly 

 thereafter (1864) Huxley redivided and renamed his 

 groups, characterizing the Annulosa as having a double 

 chain of ventral ganglia and including therein the Ar- 

 thropoda (== Articulata) and Annelida, leaving those 

 without such a nervous system, Echinodermata and 

 Scole ida, in the Annuloidea; he listed the classes Rotifera, 



Turbellaria, Trematoda, Taeniada (Cestoda), Nematoidea, 

 .'Acanthocephala, and Gordiacea as comprising the Sco- 

 lecida. On the basis of presence or absence of external 

 ciliation he put forward, in 1878, the subdivisions Trich- 

 oscolecida for the first four classes and Nematoscolecida 

 for the remaining classes plus the Gastrotricha despite 

 ciliation in some members of the latter. 



The most conspicuous feature in all discourses on 

 nemic relationships from the time of Huxley to the 

 present, is that the Nemathelminthes concept (including 

 classes Nematoda, Acanthcsaphala, and Gordiacea or 

 Nematomorpha) is not and never has been accepted by 

 comparative anatomists though it is seemingly entrenched 

 in zoologic literature. 



Without attempting to go into detail, we shall outline 

 the general theories with the points brought forward by 

 the various authors, then give a comprehensive descrip- 

 tion of the primitive nematode, followed by an evalua- 

 tion of the interrelationships of the various groups. 



1. ECHINODERMATA-SCOLECIDA THEORY. This grouping, 

 originating with Huxley (1856, -64, -78), received its 

 only support frcmi Bastian (1866) who saw the Nematoda 

 as bridging the gap between the echinoderms on one 

 side and through Acanthocephala to other scolecids on 

 the other side. This theory was based entirely on the 

 homology of the ambulacral and water-vascular systems. 

 Though the ambulacral system opens on the dorsal side 

 in many echinoderms (Holothuroidea and Crinoidea ex- 

 ceptional) and the water-vascular system on the same 

 side in rotifers, the corresponding system opens on the 

 ventral side of nematodes. Bastian states "for pur- 

 poses of transcendental anatomy" it makes no difference 

 which side is dorsal as long as the other organs (anus, 

 reproductive openings) retain their relative positions. 



2. Annelid-Chaetognath-Nemathelminth Theory. 

 The earth-worm-ascarid comparison is by far the most 

 natural since here two of the oldest known worms were 

 originally considered congeneric and were subsequently 

 separated. In 1866 Schneider classified all Vermes, as 

 he knew them, on the basis of musculature. These groups 

 were: 



1. Nemathelminthes — skin and muscle tissue of body wall 

 in two layers. 



1. With one layer of longitudinal muscles, or two 

 layers, the outer being circular and the inner 



longitudinal; lateral chords absent in latter. 



(a) Unsegmented. Longitudinal muscles only. 



Nematoidea 



Chaetognatha 

 ('b) Segmented 



(1) Longitudinal fibres only, median chords only 

 — Gymnotoma (for Ramphogordius "a seg- 

 mented Gordius"). 



(2) Longitudinal and transverse fibres — Chaeto- 

 poda. 



2. Inner longitudinal and outer circular muscles; 

 lateral chords absent. 



Acanthocephala 

 Gephyrea 



II. Plathyhelminthes — Muscle fibres embedded in skin 

 ti.ssue, longitudinal, circular, and sagittal all one layer. 



1. Oblique cross fibres absent. 



Trematoda, Dendrocoela, Hirudinea, Onycophora. 



2. Oblique cross fibres absent. 



Cestoidea, Rhabdocoela. 



As Schneider later (1873) stated, he viewed Lumbricus 

 and Nereis as annulated ascarids just as he considered 

 PoJycjordius an annulated Gordius. The idea of chaetog- 

 nath-nemathelminth relationships has only persisted in 

 a few of the more archaic textbooks and we understand 

 the chaetognaths are now regarded by many workers 

 as having lower chordate relationships. Biitschli (1875) 

 pointed out that nematodes are embryologically unseg- 

 mented while Sagitfa (Chaetognatha) is composed of three 

 segments. 



Vejdovsky (1886) proposed the group named Nema- 

 tomorpha for the Gordiacea and the odd marine 

 genus Neetonema and related the whole class to the an- 

 nelids rather than to the nematodes. J. Thiel (1902) was 

 particularly impressed by Nematomorpha-"annelid" re- 

 lationships pointing out that the female solenogaster, 

 Neomenia, has two posteriorly opening gonocoeles into 

 which numerous germ sacs empty. Such relationship was 

 apparently approved by Rauther (1909) who stated that 

 the only similarity between nematod;? and gordiids is 

 that both are long thin worms. For the acanthocephalans 

 Rauther suggested gephyrean relations dwelling to con- 

 siderable extend upon Sipuneidiis. As will be seen later, 

 Rauther viewed nematodes as primarily related to arthro- 

 pods. 



Greeff (1869) was so impressed by the secondary and 

 superficial segmentation of Desvioscolex that he felt this 



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