tary form with an H-shaped excretory system, valved 

 esophageal bulb, caudal glands, and having vulva, anus, 

 and excretory pore opening together (a separate orifice 

 of the excretory pore somewhat posterior to the posi- 

 tion in present day nematodes was considered as a possi- 

 bility). He considered the mixture of radial and bilateral 

 symmetry in nenias as due to their change from a mobile 

 to a semi-sessile life. Bilaterality is assoiiated with the 

 mode of locomotion (dorso-ventral oscillation) of nema- 

 todes and radial symmetry with a sessile mod'i of life. 

 Bilaterality occurs in the excretory system, musculature, 

 and chords while radial symmetry occurs in the lips, 

 mouth, and esophagus. As he conceived it, the appear- 

 ance of radial symmetry coincided with the loss of cilia 

 in the anterior part (corpus) of the esophagus and 

 occurred when the original nematode was formed. This 

 was a semi-sessile form. A plurality of present day free- 

 living nematodes are partially sessile and when they 

 move it is on the longitudinal axis; neither side is flat- 

 tened for they do not normally rest in a prone position; in 

 a dish of water they lie on their side abnormally. Ap- 

 parently, he hypothesized, there was a primary motility, 

 a secondary sessility (at origin of nematodes) and a 

 tertiary remotility (within the Nematoda). Only this 

 hypothesis could explain the mixtures of symmetries 

 known to occur. Comparing- the primitive nematode with 

 the rotatorian he found it to conform in cuticle, hypoder- 

 mis, presence of primary body cavity, divisions of gut 

 (anterior, middle, and posterior), presence of caudal 

 glands, original round form of body, and that possibly 

 the musculature traces back to common ancestry. He con- 

 sidered the possibility of an homology of the dorsal side 

 of the rotatorian with the ventral side of the n = mic 

 body, but rejected this hypothesis because of the embry- 

 ology. The ventral side cori'esponds to the open side of 

 the gastrula, identical in both groups. Measured from 

 the posterior, the relative positions of the anus, repro- 

 ductive and excretory system orifices are the same in the 

 two groups; hence the orifice of the excretory system 

 could first have separated from the cloaee and thereafter 

 the vulva of the female separated from the gut orifice. 

 The variability in position of the vulva was cited as evi- 

 dence of recentness of its separation. Paired parallel 

 gonads were considered primitive for both sexas although 

 no such example is known in free-living nemas. Ciliation 

 of the anterior gut was considered primary; the mastax 

 homolog^ized with the esophageal bulb of Rhabditis, radial 

 symmetry developing from bilateral. The amphids and 

 accompanying glands wei'e homologized with the retro- 

 cerebral organ and subcerebral glands of Callidhm. On the 

 whole, his comparison seems apt but the mastax is actual- 

 ly triradiate in symmetry, secondarily bilateral, and 

 the ciliated anterior gut of rotatorians is a recent acquisi- 

 tion (secondary invagination), the primary stomodeum 

 forming the mastax and esophagus (= esophago-intestinal 

 valve of nemas). 



More recently Remane (1928, 1929) has expressed the 

 view that gastrotrichs are near the ancestor of both 

 nematodes and echinoderids and that they in turn devel- 

 oped from archiannelid-like (trochophore) progenitors. A 

 trochophore origin was also suggested for rotatorians but 

 the unsuitability of the Trochhelminthcs (Zelinka in- 

 cluded rotatorians, gastrotrichs and echinoderids) as a 

 systematic group was pointed out. Rotatorians differ 

 from gastrotrichs and echinoderids, as well as from 

 nematodes, by having circular muscles, a ciliated fore- 

 gut, and bilateral mastax all of which are secondary 

 developments. Remane subscribed to the Asche'minthes 

 concept. 



The Primitive Nema 



Having brought forward the various theories to ac- 

 count for nemic origin we need examine them in the 

 light of present knowledge. In order to do this a picture 

 should be formed of the primitive nema. Considering the 

 various types of evidence customarily available we find 

 some forms of knowledge conspicuous by their absence, 

 others by their richness. 



Geology. Taylor (1935) reviewed the knowledge of 

 fossil nemas and listed a total of six species of which 

 two were mermithids while the remainder were free- 

 living nemas. The mermithids [Hcydoniiis antiquus (v. 

 Heydon, 1862) and H. matutinvs (Menge, 1866)] were 



collected from Rhine lignite (Eocene) and Baltic amber 

 and the free-living nenias [Oligoplectus succini (v. Duis- 

 burg, 1862), Vetus duisburgi Taylor 1935, V. pristinvs 

 (Menge, 1866) and V. capillacciis (Menge, 1866)] were 

 all found in Baltic amber (lower Oligocene). As no 

 morphologic details can I'oe distinguished the records 

 are of no advantage from that standpoint; neither are 

 they of value in determining the origin of nemas since 

 the group obviously must predate the Tertiary. These 

 records, however, help us to understand why fossil nemas 

 are so rare. The nema has no hard parts such as in- 

 sects have for preservation and a nema must be included 

 in a deposit such as rosin or something of exceedingly 

 fine grain. Repeated examinations of fossil shales and 

 ambers by the writer have been uniformly negative. 



Fig. 142. 



Fossil nemas. A — Heydonlus anttguus; B-C — Heyrlanius matuti- 

 tins; D — Vetus prislhius; E — Velus capillaceus; F — Oligoplectus 

 succini: G-H— Vetus duisburgi. Alter Taylor, 1935. Proc. Helm. 

 Soc. Wash. V. 2 (1). 



DiSTKiDUTlON. Among the free-living nemas there seems 

 to be little correlation between geographical locality and 

 species. Both saprozoic and predatory forms are more 

 specialized as to medium than continent. In some fresh- 

 water groups Thome (1929) has found European species 

 common on the western mountain peaks in this country, 

 while those associated with decay may be iconsidered 

 universal. Identical marine species have likewise been 

 collected from European and American Atlantic coasts 

 though this parallel occurrence is less common. Genera 

 amon.g free-living nemas are certainly worldwide. In 

 parasitic nemas, the parasites distribution is usually 

 coincident with that of the host. 



The parasites of man have received considerable at- 

 tention from the standpoint of distribution. The "Ameri- 

 can'' hookworm, Necator americanus, is distributed over 

 a large part of the earth and it could easily have 'oeen 

 imported to the Americas with negroes from Africa (Fig. 

 143). Two filariids of man, Wnchereria bancrofti and 

 Dijyetaloncma pcrF:tans (syn. Acanthocheilonema perstans) 

 seem to have originated in Europe or Asia and later to 

 have been brought to the Western World (Fig. 144). 



Evidence of the origin of physiological races of a spe- 

 cies in two or m')re hosts (ex. Ascans lumbricoides in pig 

 and man) may indicate at least one line of evolution. 



193 



